Geotechnical Reports in Australia: The Complete Developer's Guide

A geotechnical report may be the most underpriced line item in a typical Australian feasibility study, and the most expensive one to ignore. A $600 residential soil test or a $60,000 high-rise site investigation could determine whether a project clears its hurdle rate or collapses into an underpinning claim, a Subsidence Advisory NSW approval delay, or a Class P reclassification that adds six figures to substructure cost.

This guide covers what developers may need to know about geotechnical reports across all Australian states and territories, from desktop studies through to high-rise site investigations. It walks through the regulatory framework, the investigation process, how to read a report, real 2025 cost ranges, state-by-state variations, adjacent reports developers often bundle, and the red flags that can kill a feasibility study before construction starts.

What a geotechnical report actually is

A geotechnical report is typically understood as a professional engineering document that characterises the subsurface conditions at a site (soil, rock and groundwater) and converts that data into design and construction advice. AS 1726:2017 Clause 5 generally requires that “a geotechnical model shall be developed for every geotechnical site investigation”, and that model is typically the intellectual spine of the report.

A competent interpretive report tends to do six things at once. It describes the ground profile and stratigraphy from boreholes, cone penetration tests and test pits. It synthesises that data into a geotechnical model, which could be understood as a three-dimensional interpretation of how the layers behave. It derives engineering parameters such as bearing capacity, settlement, shear strength, permeability and reactivity. It describes the groundwater regime (perched, unconfined, confined, tidal influence) and its seasonal variation. It identifies geohazards including acid sulfate soils, expansive clays, karst, mine subsidence, landslip and liquefaction potential. And it translates all of that into recommendations for footings, earthworks, excavation support, pavements, piling and dewatering.

The report is typically the defensible evidence base for the structural engineer’s footing design, for the builder’s construction methodology, for the certifier’s compliance decision, for the insurer’s warranty cover and, increasingly, for the developer’s own feasibility model.

The five common report types

Preliminary or desktop studies review historical aerial photography, published geology, planning certificates, previous DA lodgements, contamination registers and soil data. They typically cost $1,500 to $4,000 and take one to two weeks. Their value may be significant at the pre-offer stage.

Detailed Site Investigations (DSI) are generally the primary product. These include boreholes, CPTs, test pits, in-situ and NATA-accredited laboratory testing, interpreted to AS 1726:2017. This is typically what a developer could commission for anything beyond a single dwelling.

Factual reports present raw data only (borehole logs, laboratory test certificates, location plans) with no interpretation. They may be standard on major infrastructure tenders where the principal wants bidders to do their own interpretation.

Geotechnical Interpretive Reports (GIR) or Foundation Recommendations Reports interpret the factual data into design parameters, allowable bearing pressures, pile capacity curves and footing recommendations. These could be understood as the reports developers most often confuse with “the geotech report”, as they are usually issued alongside or after the factual report.

Site classification reports to AS 2870 are typically the cheapest product. These are narrow residential deliverables that report a single site class (A, S, M, M-D, H1, H2, E or P) and footing recommendations. They are generally not a substitute for a DSI on anything larger than a detached house.

Why Australia requires them

Geotechnical reports may be mandated through a layered architecture of obligation. The National Construction Code (NCC) references AS 2870 for Class 1 and 10 buildings, and AS 2159, AS 3600 and the AS 1170 series for larger structures. Consent authorities impose site-specific requirements under the Environmental Planning and Assessment Act 1979 (NSW), the Planning and Environment Act 1987 (VIC) and the Planning Act 2016 (QLD).

Lenders and builders’ warranty insurers, including HBCF in NSW, DBI in Victoria and QBCC Home Warranty in Queensland, generally will not cover residential work without a site classification and footing design. The Model WHS Regulations typically require geotechnical advice for excavations over 1.5 metres and for tunnelling. Since 1 July 2021, the NSW Design and Building Practitioners Act 2020 has made geotechnical engineering a regulated design for Class 2 buildings, with statutory duty-of-care consequences running ten years under section 37.

Who actually prepares them

The signatory should typically be a geotechnical engineer (a civil engineer specialised in soil and rock mechanics) or an engineering geologist. Credentials worth verifying could include CPEng (Chartered Professional Engineer via Engineers Australia), NER listing, and state registration where required.

RPEQ registration is generally mandatory in Queensland under the Professional Engineers Act 2002, administered by the Board of Professional Engineers of Queensland, for any professional engineering service performed in or for Queensland, even by an interstate engineer. Victoria’s RPEV scheme has been mandatory since 1 July 2021 under the Professional Engineers Registration Act 2019. NSW requires Design and Building Practitioner registration for regulated designs on Class 2 work.

The Australian Standards and regulatory framework

AS 1726:2017, the investigation standard

AS 1726:2017 (published 2 May 2017, replacing the 1993 edition) is typically the central standard for geotechnical site investigations. Although it is not directly called up by the NCC, it could be considered effectively mandatory through DA conditions, consultant standard of care, and the expectations of structural engineers, certifiers and insurers.

The 2017 version introduced risk-based investigation categories adopted from Eurocode 7:

• GC1 applies to small, simple structures on known good ground
• GC2 applies to conventional structures on typical ground, including most buildings, small bridges and retaining walls
• GC3 applies to unusual or complex structures, soft or unstable ground, or projects with high consequences of failure, such as tall buildings, dams and tunnels

Investigation effort, personnel experience and reporting rigour typically scale with category.

Significant 2017 changes included a redefined soil fines boundary. More than 35% passing 75 micrometres now means a fine-grained soil, where it was previously 50%. A secondary-constituent threshold classifies a soil as “clayey SAND” with just 12% clay. Rock logging dropped the Extremely Low strength category, so EL material is now logged as soil with rock fabric noted. Moisture descriptors were standardised.

AS 2870-2011, the residential classification standard

AS 2870-2011 governs residential slabs and footings for Class 1 and 10 buildings and is typically mandatory via NCC Volume Two. Its classification table ties site class to characteristic surface movement (ys), which could be understood as the vertical surface movement with less than a 5% exceedance probability over the 50-year design life.

Appendices D and E contain the ys computation method and the Thornthwaite Moisture Index (TMI) climate zones that typically set the depth of design suction change (Hs). Appendix H addresses trees and abnormal moisture conditions.

AS 3798-2007, AS 1289 and AS 2159

AS 3798-2007 sets three inspection and testing levels for earthworks. Level 1 is full-time independent supervision required for controlled fill under buildings and to support re-classification under AS 2870. Level 2 is part-time with statistically-based test frequencies. Level 3 is minimum testing for low-risk areas. AS 3798 is technically a guideline, but councils routinely impose it through DA conditions.

AS 1289 is the sixty-plus-method soil testing suite, covering Atterberg limits, linear shrinkage, particle size distribution, compaction, CBR, triaxial testing, consolidation, permeability and the shrink-swell index (Iss). Laboratories may typically need to be NATA-accredited for results to be defensible.

AS 2159-2009 governs piling. Section 2 makes a piling-specific site investigation mandatory. Section 4.3 derives the basic geotechnical strength reduction factor from an Individual Risk Rating covering geology, investigation quality, design method, installation and load testing, giving values from 0.40 to 0.76.

NCC 2022 and the 2026 transition

NCC 2022 took effect in most states from 1 May 2023, with Amendment 1 applying from 1 May 2024. NCC 2025 has been delayed: Building Ministers’ October 2025 communiqué confirmed publication by 1 February 2026, with adoption available from 1 May 2026. Volume One Part B1 references AS/NZS 1170, AS 2159, AS 2870, AS 3600, AS 4678 and AS 5100.3. Volume Two Part H1, with the ABCB Housing Provisions Parts 3 and 4, makes AS 2870 classification mandatory for Class 1 and 10 buildings.

State planning and council overlays

NSW applies geotechnical obligations through LEPs and DCPs under the EP&A Act 1979, plus SEPP (Resilience and Hazards) 2021 for contamination and acid sulfate soils. Councils with dedicated geotechnical clauses include Northern Beaches, Blue Mountains, Wollongong, Central Coast (DCP 2022 Chapter 3.7), Woollahra and Wollondilly.

Victoria typically triggers geotechnical reporting via the Erosion Management Overlay (Clause 44.01), Salinity Management Overlay (44.02) and the Land Subject to Inundation Overlay. Queensland operates under the State Planning Policy and council DCPs, with Ipswich City Plan’s Mining Influence Area overlays triggering mining and geotechnical studies for any DA over historic coal workings.

Inside the investigation: step by step

Desktop and walkover

Every competent investigation typically starts before anyone puts a drill rig on site. The desktop study collates published geology from Geoscience Australia, MinView NSW, GeoVic, QDEX, SARIG and GeoVIEW.WA. Historical aerial photography (1943 to 1974) could be accessed through SIX Maps, Landata and Landgate. Section 10.7 certificates in NSW, BoM rainfall and TMI data, bore and groundwater databases and previous geotechnical reports held by councils are all standard inputs.

The walkover assesses topography, tension cracks, trees with curved trunks, hummocky ground, seepage, surface outcrops, existing cracking on structures, vegetation indicators, drainage patterns, and access for plant. It is also when Before You Dig Australia clearance is initiated, which could take three to ten business days.

Intrusive investigation techniques

Australian practice typically uses a standard palette of techniques.

• Solid flight auger drilling is cheap and fast for shallow work in cohesive soil above the watertable, typically 75 to 150 mm diameter to six or ten metres
• Hollow stem auger acts as its own casing and allows SPT and undisturbed sampling through the centre, which may be essential for monitoring wells in loose sand and saturated soils
• Rotary wash and mud rotary enable deep boreholes in variable soils below the watertable, with polymer or bentonite stabilising the hole
• Diamond core drilling produces continuous rock core for RQD, defect logging and UCS or point-load testing, which could be essential for bored-pile rock socket design in Sydney sandstone or Melbourne basalt
• Sonic drilling is increasingly used for variable fill and contamination work
• Percussion and RC drilling may be needed to punch through boulders and hard fill

The Standard Penetration Test (AS 1289.6.3.1) is typically the most common in-situ test. It uses a split-barrel sampler with a 63.5 kg hammer and a 760 mm drop, with the N value counted over the final 300 mm of a 450 mm drive. Refusal is recorded where 50 blows fail to advance 150 mm. Australian hammer efficiency is typically around 60%.

N values map to soil density or consistency. Loose sand is typically below N=10, medium dense is N=10 to 30, dense is above 30. For clays, soft is below N=4, firm is N=4 to 8, stiff is N=8 to 15, and hard is above N=30. N values may feed empirical correlations for friction angle, modulus, and shaft and end-bearing parameters in AS 2159 pile design.

The Cone Penetration Test (CPT and piezocone CPTu) pushes an instrumented 10 cm² cone at 20 mm per second, measuring tip resistance, sleeve friction and pore pressure. It typically produces continuous 20 mm-resolution data at 100 to 150 metres per day with no spoil. It could be unbeatable in soft to firm soils and sands but generally reaches refusal in dense sand, gravel or rock. Major Australian providers include ConeTec, Insitutek, Legion Drilling and Geotechnique.

Test pits typically log the profile directly to around three to four metres with a five-tonne excavator. WHS regulations prohibit unprotected entry over 1.5 metres, so most logging is done from the surface. The Perth Sand Penetrometer (AS 1289.6.3.3) is the WA-specific compaction-verification test for sand pads.

Groundwater is typically monitored via standpipes (50 mm Class 18 PVC with slotted screen, sand pack, bentonite seal) or vibrating wire piezometers for confined layers. Best practice may be to collect one to three months of baseline data before dewatering design.

How many holes, and how deep

AS 1726:2017 deliberately avoids prescriptive borehole-spacing numbers and instead ties effort to the Geotechnical Category. Australian practice, informed by Burt Look’s Handbook of Geotechnical Investigation and Design Tables, uses representative guidance:

• Single dwelling: two to three boreholes or DCPs to 1.5 times the expected footing depth or to refusal on rock
• Residential subdivision: one borehole per two to four lots, or a 30 to 50 m grid, with at least one deep hole per hectare
• Commercial buildings of 1,000 to 10,000 m²: a 20 to 40 m grid with one borehole per corner and major column line
• High-rise and GC3 projects: a 15 to 30 m grid with a deep hole per corner and centre, extending at least 1.5 times the basement width below founding level
• Warehouses: one borehole per 2,500 to 5,000 m² plus DCPs at 20 to 30 m centres for slabs
• Road investigations: typically follow Austroads at 100 to 250 m spacing

Investigation depth scales with load. A single-storey dwelling may need around 3.5 metres, while a five-storey building could require at least 24 metres or three metres of rock proof core. Piles generally need five pile diameters below toe level plus three metres of rock proof core for end bearing. Basements may need one to 1.5 times excavation depth below base.

Laboratory testing

The routine suite for a development project typically includes moisture content, particle size distribution with hydrometer, Atterberg limits, linear shrinkage, shrink-swell index (for direct input to ys calculations), Emerson dispersion class, standard and modified compaction, CBR, direct shear or triaxial testing, consolidation, permeability, and pH, EC, chloride and sulfate testing for concrete aggressivity per AS 2159 and AS 3735. Rock UCS or point load testing follows AS 4133.

Turnaround from a NATA-accredited lab is typically two to four weeks, and up to six weeks for full consolidation and triaxial suites.

Timeline

A single residential classification typically takes three to seven business days in metro areas. A standard commercial or medium-density DSI generally runs four to eight weeks from engagement to final report, with drilling availability usually the bottleneck. Complex sites (deep basements, landslip, contamination, reclaimed land, Sydney sandstone interfaces) could take eight to twelve weeks, often in phases. Major infrastructure investigations may run three to six months.

Reading the report: what each section means

A competent AS 1726:2017 interpretive report typically follows a predictable structure, and developers who know what to look for in each section could catch problems in the quote stage rather than the construction stage.

The executive summary should tell you the site classification, the recommended foundation, and any show-stoppers (rock excavation, acid sulfate soils, landslip, contamination) in plain language on one to two pages. If it does not, that could itself be a warning sign.

The scope and introduction defines what was commissioned, which matters enormously for reliance. A report scoped for a two-storey dwelling is generally not fit for a six-storey apartment with basement.

The site description and history covers cadastre, topography, surrounding land use, historical uses informed by aerial photography, and regulatory overlays. The regional geology typically references the relevant 1:100,000 or 1:250,000 sheet and the stratigraphic units expected (Hawkesbury Sandstone and Ashfield Shale in Sydney, Newer Volcanics basalt and Coode Island Silt in Melbourne, Brisbane Tuff and Neranleigh-Fernvale beds in SEQ).

The fieldwork section documents dates, rig types, drilling fluids, logger qualifications and locational accuracy. The subsurface conditions appendix carries the borehole logs to AS 1726:2017 format: depth, graphic log, unit description (primary constituent in capitals, secondary constituents, colour, plasticity, moisture, origin, formation), strength or density, defects, sampling, groundwater and drilling notes.

The groundwater conditions section reports depths on completion and after standing time, inferred seasonal range, perched vs unconfined vs confined systems, seepage zones and likely dewatering inflows. This is typically the section that drives basement feasibility in Sydney CBD, Mascot, Alexandria, Parramatta, Port Melbourne and Fortitude Valley.

The engineering discussion derives allowable bearing pressures. Australian practice typically quotes net pressures at serviceability with a factor of safety around 2.5 to 3 on ultimate. Typical presumptive ranges could include:

• 50 to 100 kPa in firm clay
• 100 to 200 kPa in stiff clay or medium dense sand
• 200 to 400 kPa in very stiff clay or dense sand
• 300 to 600 kPa in extremely weathered rock
• 600 to 1,500 kPa in highly weathered rock
• 1,500 to 5,000 kPa in moderately weathered to fresh Sydney sandstone or shale
• Fresh basalt or granite could carry 5,000 to 10,000 kPa subject to defects

Settlement predictions typically cover immediate, consolidation and creep components. Common criteria are one-in-500 differential for framed buildings, one-in-300 for warehouses and 20 to 25 mm total for residential slabs.

Earthworks recommendations follow AS 3798. Controlled fill typically requires at least 98% of Standard MDD and moisture within plus or minus 2% of OMC for cohesive fill, or at least 75% density index for granular fill.

The limitations and assumptions section defines what the report does not cover. Contamination, asbestos, unexploded ordnance and seasonal groundwater variation are typically explicitly excluded. It is worth reading this section before relying on the report.

AS 2870 classifications in detail

The AS 2870 site classification may be the most referenced and most misunderstood output of Australian geotechnical work. The classification typically drives slab stiffness, edge beam depth, reinforcement, articulation and, often, whether a builder’s fixed-price contract survives contact with the actual ground.

Class A describes stable sand or rock with effectively zero movement. It tends to dominate the Perth metropolitan area on the Swan Coastal Plain (Quindalup, Spearwood and Bassendean dunes), the Sydney sandstone belt (North Shore, Eastern Suburbs, Northern Beaches), and parts of Hobart and Darwin. Shallow strip or pad footings to AS 2870 deemed-to-comply designs typically apply, with nominal reinforcement and shallow edge beams of about 300 mm. This could be considered the baseline cost.

Class S (ys 0 to 20 mm) covers silty clays of low plasticity and sandy clays, including coastal NSW estuarine plains, much of SEQ lowlands, and sandy-clay transition zones around Perth and Adelaide. Standard slab-on-ground with modest 300 to 400 mm edge beams is typical. Waffle pods are generally acceptable. The premium over Class A is typically minimal.

Class M and M-D (ys 20 to 40 mm) is commonly the national default. It describes medium-plasticity clays, Quaternary alluvial clays, weathered shales and residual clays from mudstone. Most of Brisbane and Ipswich, large tracts of outer Sydney (Blacktown, Penrith, The Hills), Adelaide plains and middle-ring Melbourne tend to fall in this class. M-D applies in drier interior climates where Hs is at least 3 metres. Additional cost over Class A is typically zero to $3,000 on a detached house.

Class H1 (ys 40 to 60 mm) describes high-plasticity clays of moderate depth, including weathered basaltic clays, Keswick Clay in Adelaide, Melbourne western-suburb basaltic clays and Hunter Valley clays. Western and northern Melbourne (Truganina, Sunbury, Craigieburn, Hoppers Crossing), western Sydney clay belts, much of metropolitan Adelaide, inner-west Brisbane and Toowoomba tend to fall here. Design implications typically include deeper edge beams (500 to 700 mm), tighter internal beam spacing, upgraded mesh, and articulation joints. The cost premium is typically $5,000 to $15,000 over M-class on a standard 200 to 250 m² home.

Class H2 (ys 60 to 75 mm) describes very high-plasticity clays including black earths and deep Quaternary basaltic residuals. Inner Adelaide heritage suburbs (Wayville, Westbourne Park, Goodwood, Netherby), Mount Lofty Ranges, outer western Melbourne (Melton, Rockbank), parts of Maitland and the Darling Downs cracking-clay regions tend to fall here. The 2011 split of the old “H” class into H1 and H2 exists specifically to force stiffer designs on these soils. Typical design involves stiffened raft with 700 to 900 mm edge beams. The cost premium is typically $10,000 to $25,000 over M.

Class E (ys greater than 75 mm) describes extremely reactive smectite clay, including deep black vertisols, Keswick Clay at depth, and Maitland-Muswellbrook deep cracking clays. Pockets of inner Adelaide, Darling Downs, parts of western Victoria, and the Liverpool Plains may fall here. Standard designs are generally inadequate, and an engineered stiffened raft with 900 to 1,200 mm edge beams or a pile-and-slab system with bored piers or screw piles founded below the active zone is typically required. Cost premium is typically $15,000 to $40,000 over M.

Class P is the problem site, outside the standard ys method. Triggers typically include:

• Soft or loose soils
• Landslip
• Mine subsidence (Newcastle, Ipswich, Wollongong)
• Collapsing soils
• Erosion-prone coastal dunes
• Uncontrolled or deep fill
• Reactive sites subject to Abnormal Moisture Conditions per AS 2870 Appendix H, such as trees within one to 1.5 times mature height, dams, leaking services or poor drainage

Class P is typically outside the scope of AS 2870 standard designs and requires specific engineering. The cost premium is highly variable. On complex P sites with landslip or mine subsidence, total foundation premium could exceed $50,000 to $100,000.

The ys computation

AS 2870-2011 Appendix D gives the formal calculation. Surface movement is typically the sum over the depth of design suction change of the layer instability index, suction change, layer thickness and a lateral restraint factor. The shrink-swell index from AS 1289.7.1.1 is generally the preferred Australian input.

Hs is set by Thornthwaite Moisture Index climate zone. Brisbane and Ipswich typically sit at 1.5 to 2.3 metres, Sydney metro around 1.8 metres, Melbourne metro 1.8 to 3.0 metres (the western fringe is now climate zone 4), Adelaide 4.0 metres under the standard, and arid inland over 4 metres.

A critical developer insight is that cut and fill may dramatically change ys. AGS research has documented Newcastle sites with natural ys of 35 to 40 mm (M-class) rising to 50 to 80 mm (E-class) after filling. Volume builders who assume M-class without a site-specific calculation under Appendix D could be significantly wrong.

Foundations: how geotech drives substructure cost

Soil classification typically drives six design parameters: raft stiffness, edge and internal beam depth, beam spacing, reinforcement sizing, masonry articulation, and pier embedment. On a 200 to 250 m² detached home, translated substructure costs may run approximately:

Waffle pod vs stiffened raft is typically the most common residential substructure decision. Waffle pods sit on ground using polystyrene pods to form voids, typically using around 30% less concrete and 20% less steel than a conventional raft. Waffle is generally faster (one to two days for a 200 m² slab), suited to flat Class A, S and M sites, and marginal on H1. Stiffened raft is commonly the preferred engineering choice for H2 and essentially mandatory for E because of greater stiffness and lower termite risk.

Screw piles vs bored piers is typically the most common deep-foundation decision. Screw piles install hydraulically with torque monitoring to AS 2159, produce no spoil, handle tight access and wet weather, and are same-day load. Typical residential 85 kN piles cost $150 to $300 each in Melbourne. Bored piers need a larger drill rig and produce substantial spoil but are easy to upsize in diameter. Equivalent residential cost is typically around $400 to $700 each. Bored piers tend to dominate volume housing where access and spoil are not issues, while screw piles are often the default for reactive-clay retrofit, P-sites and tight access.

For basements in rock cities, soldier pile walls with shotcrete lagging are typically the most cost-effective approach at $1,500 to $3,000 per m² of wall face. Contiguous piled walls run around $1,800 to $3,500 per m², secant walls where water cut-off is required are higher, and ground or rock anchors could cost $3,000 to $8,000 per anchor depending on length and capacity.

What it actually costs in 2024 and 2025

Published fee schedules for Australian geotechnical work are rare, as most firms typically quote per project. The ranges below draw on published rates from Ideal Geotech, Smolders Geotechnical, Fortify Geotech, iSeekPlant cost guides, government tender records and industry benchmarks.

Residential single dwelling

A basic AS 2870 site classification in Greater Melbourne may run $390 including GST to $1,200 plus GST, typically $500 to $800 plus GST. In Sydney, NSW, QLD and ACT the range is typically $700 to $1,500, generally $900 to $1,200. Extensions and upper-storey additions could add $150 to $600 per footing probe. A full residential DSI for a basement, pool or complex site typically runs $2,500 to $8,000 or more, generally $3,000 to $6,000. Hillside sites with slope-stability and AGS risk assessment could add $1,000 to $6,000.

Multi-unit, medium and large developments

Duplex and dual-occupancy soil work typically runs $1,200 to $2,500. A townhouse development of four to ten units may cost $3,500 to $8,000. A DSI for a four to six storey apartment with one to two level basement typically runs $15,000 to $45,000, with six to ten boreholes to 15 to 25 metres, SPT, lab suite and dewatering assessment. A high-rise CBD tower DSI typically runs $60,000 to $250,000 or more, driven by deep cored boreholes (30 to 60 metres into Sydney sandstone or Melbourne basalt), packer testing and monitoring. Peer review fees are typically 10 to 20% of the original fee.

Industrial and commercial

A warehouse of around 400 m² footprint may cost $5,000 to $12,000. Single-storey office or retail typically runs $4,000 to $10,000. Large distribution centres over 5,000 m² could run $25,000 to $80,000.

Cost drivers and unit rates

Drill rig with two-man crew runs about $190 per hour. Air rotary boring is typically $148 to $197 per metre, and diamond rock coring $197 to $262 per metre. Local mobilisation is typically $450 per job, while interstate mobilisation runs around $900 per day. A truck-mounted auger rig day rate is typically $2,000 to $3,000.

Cost per fully-loaded borehole could work out as:

• $250 to $500 for shallow auger under three metres
• $1,800 to $3,500 for a standard SPT borehole to ten metres
• $4,500 to $8,000 for a cored rock borehole to 20 metres
• $8,000 to $15,000 or more for a deep cased borehole with packer or monitoring well

CPT pricing is largely commercial-in-confidence. Indicative rates are typically $25 to $45 per metre pushed or $2,500 to $4,500 per day plus $500 to $1,500 metro mobilisation. Geophysical surveys (MASW, seismic refraction) typically run $3,000 to $8,000 per day.

Laboratory testing costs may include:

• $40 to $80 for moisture content
• $400 to $800 per sample for Atterberg limits
• $200 to $400 for PSD with hydrometer
• $250 to $450 for Proctor compaction
• $450 to $750 for 4-day soaked CBR
• $300 to $500 for shrink-swell index
• $700 to $1,200 for multi-stage consolidation
• $400 to $700 for UU triaxial
• $900 to $1,800 for CU triaxial with pore water pressure
• $450 to $750 for direct shear
• $550 to $1,100 per sample for contamination suites

Charge-out rates

Typical 2024 and 2025 consulting charge-out rates could be understood as:

• $140 to $180 per hour for graduates
• $180 to $230 per hour for mid-level (three to six years)
• $220 to $290 per hour for senior engineers
• $300 to $450 per hour for associates and principals
• $400 to $550 per hour for technical directors
• $120 to $160 per hour for field technicians

Regional variations

Sydney metro is typically 10 to 20% above Melbourne due to sandstone coring and basement work. Brisbane and Adelaide sit near baseline. Perth is typically baseline to slightly below, as sandy soils drill quickly. Regional NSW, VIC and QLD add 10 to 25%, while regional WA, outback SA and far North Queensland could add 30 to 60%. Darwin adds 20 to 40%, with Arnhem and Barkly regions at 60 to 100%. Tasmania adds 15 to 30% due to limited rig supply. FIFO mining sites could run 50 to 150% above base with 12-hour day rates.

Australian geotechnical spend should typically be 0.3 to 1.0% of total project cost for buildings, higher for earthworks and infrastructure. Douglas Partners has documented how under-investing at 0.1% could be a false economy, with international studies consistently linking inadequate site investigation to significant cost overruns.

With Feasly’s feasibility software, you can model different substructure scenarios and their impact on IRR, helping you size contingency appropriately based on geotechnical risk category.

State-by-state variations

New South Wales

Sydney Basin geology is typically defined by Hawkesbury Sandstone and Ashfield and Mittagong Shale. The Pells, Mostyn and Walker (1998) 5-class rock mass classification is commonly the de facto design tool for bored piers. Botany Sands create dewatering and liquefaction challenges in Mascot, Alexandria and Rosebery. Coastal estuarine clays around Parramatta, Cooks, Georges and Hawkesbury Rivers are typically soft, compressible and often acid-sulfate. Blue Mountains, Northern Beaches and Wollongong escarpments have systemic landslip risk.

NSW regulation typically runs through SEPP (Resilience and Hazards) 2021 Chapter 4 (contamination, replacing SEPP 55) and local ASS LEP clauses, the Acid Sulfate Soils Manual 1998 with five planning classes, and the Coal Mine Subsidence Compensation Act 2017 administered by Subsidence Advisory NSW. Mine subsidence approval is typically required pre-DA across proclaimed districts including Newcastle, Lake Macquarie, Cessnock, Maitland, Singleton, Muswellbrook, Lithgow, Wollongong and Campbelltown.

Section 88B instruments under the Conveyancing Act 1919 are typically the primary vehicle for imposing geotechnical covenants on title, such as minimum pier depths, drainage obligations and no-build zones over sandstone cliff lines.

Victoria

Newer Volcanics basalt typically defines Melbourne’s northern and western suburbs, offering good bearing but highly reactive Class H to E residual clays with floaters and corestones that could ruin bored-pier economics. Coode Island Silt dominates the Yarra Delta (Docklands, Port Melbourne, Fishermans Bend, South Wharf). It is very soft to soft marine clay, up to 30 metres thick, with CBR under 1% and secondary compression potentially up to 10 mm per year.

Victoria typically triggers geotechnical reporting through the Erosion Management Overlay (Clause 44.01), applying extensively in Yarra Ranges, Mornington Peninsula, Merri-bek and coastal cliffs. A “Moderate” or higher property-loss risk that cannot be mitigated may block approval. Ballarat and Bendigo gold-mining subsidence is typically managed through council setbacks rather than a compensation scheme. RPEV registration has been mandatory since 1 July 2021 for engineering services including geotechnical.

Queensland

Brisbane Tuff, Neranleigh-Fernvale beds and extensive Tertiary basalt residual clays make SEQ widely Class H1, H2 or E. Brisbane’s western suburbs (Chapel Hill, Kenmore, The Gap) are typically reactive. Bowen Basin coal measures require RPEQ highwall stability and subsidence assessments. Cyclone regions (Townsville, Cairns, Mackay) in AS/NZS 1170.2 Region C and D interact heavily with footing uplift capacity. Coastal acid sulfate soils are extensive below 5 metres AHD from Cooloola to Cairns.

Ipswich Coal Measures include shallow historic underground mines from the 1840s to 1997 under Dinmore, Bundamba, Blackstone, Collingwood Park and Basin Pocket. A 2019 sinkhole at 4 Coal Street, Basin Pocket illustrated live subsidence risk. The Department of Resources Abandoned Mine Lands Program operates the Collingwood Park State Guarantee.

RPEQ registration is typically mandatory under the Professional Engineers Act 2002, which is unique to Queensland and extraterritorial. Interstate engineers designing for a QLD site typically still require RPEQ. The Queensland Acid Sulfate Soil Technical Manual v5.1 (May 2024) is the current investigation guide.

South Australia

Adelaide’s Keswick and Hindmarsh Clays are typically among the most reactive in Australia. Heritage inner suburbs (Wayville, Westbourne Park, Goodwood, Netherby, Kensington) commonly compute Class H1, H2 or E. Mount Lofty Ranges carry landslip on weathered Adelaidean metasediments (Stirling, Aldgate, Crafers). Flinders and Yorke Peninsula have sodic and saline dispersive soils.

Regulation typically runs through the Planning and Design Code (operational March 2021 across SA), the Building Rules, and the Environment Protection Act 1993 with accredited Site Contamination Auditors under Part 10A.

Western Australia

The Swan Coastal Plain has three parallel dune systems of increasing age:

• Quindalup (Holocene, loose calcareous sand, watertable often under two metres)
• Spearwood (Middle Pleistocene, yellow sands over weakly cemented Tamala calcarenite with cavities and solution pipes)
• Bassendean (Lower Pleistocene, deeply leached siliceous quartz sand with interdunal peat)

The Pinjarra Plain carries alluvial clayey floodplains, and the Darling Scarp has lateritic residual profiles over granite. Basement dewatering is typically almost universal, with DWER groundwater licences required under the Rights in Water and Irrigation Act 1914.

The Contaminated Sites Act 2003 mandates DWER reporting and classification. ASS mapping applies across the Swan Coastal Plain via DWER guidelines. The Perth Sand Penetrometer (AS 1289.6.3.3) is the WA-specific compaction test.

Tasmania

Jurassic dolerite typically dominates eastern and central Tasmania. It is very strong rock but columnar jointing creates scree and block-fall hazards. The Tasmanian Planning Scheme Landslide Hazard Code (C15.0) uses five bands: Acceptable, Low, Medium, Medium-Active and High. Mineral Resources Tasmania’s 2016 guidance operationalises AGS 2007. Declared Landslip A and Landslip B areas under the Mineral Resources Development Act 1995 cover extensive western and northern suburbs.

Australian Capital Territory

Canberra Formation Silurian mudstone and shale with weathered residual clays typically produces Class M to H reactivity. The Territory Plan 2023 (commenced 27 November 2023) governs DAs. The National Capital Plan requires NCA approval in Designated Areas. Difficult zones typically include deep fill on the reclaimed Molonglo floodplain (Denman Prospect, Coombs, Wright).

Northern Territory

Darwin’s black soil is cracking smectite Vertosols across Palmerston, Humpty Doo and Howard Springs, and is typically Class H1 to E under AS 2870 with extreme wet and dry seasonal reactivity. Coastal estuarine muds (Darwin Harbour, Shoal Bay) are typically soft, compressible and acid-sulfate. Cyclone Region C and D wind loads make footing uplift tie-down critical. PFAS contamination from Defence sites (Rapid Creek basin) is a live issue. No statutory engineer registration operates in the NT at present. CPEng and NER are the baseline, with frequent RPEQ overlay on Commonwealth projects.

Adjacent reports developers confuse or bundle

Contamination assessments typically follow the National Environment Protection (Assessment of Site Contamination) Measure 1999, amended 16 May 2013 (the “ASC NEPM”), with Schedule B1 setting Health Investigation Levels, Health Screening Levels and Ecological Screening Levels.

• A Phase 1 ESA (PSI) typically costs $3,000 to $15,000 and identifies Areas of Environmental Concern
• A Phase 2 DSI typically runs $15,000 to $100,000 or more depending on contaminants
• NSW EPA-accredited Site Auditor fees typically run $30,000 to $150,000

Co-locating contamination and geotechnical boreholes, with appropriate decontamination, may save 10 to 20% on mobilisation when planned up-front.

Acid sulfate soils investigations are typically triggered by NSW SEPP (Resilience and Hazards) 2021 Chapter 4 across five ASS classes, the Queensland SPP for land at or below 5 metres AHD disturbing over 100 m³, and DWER Swan Coastal Plain guidelines. Lime treatment rates are typically 10 to 30 kg per m³ calculated from oxidisable sulfur with a safety factor of 1.5. Investigation costs typically $10,000 to $40,000, with treatment running $50 to $200 per m³ on site, up to $300 per m³ for off-site landfilling.

Hydrogeological studies model groundwater, dewatering volumes and drawdown. Licences are typically required from WaterNSW (WAL for dewatering over 3 ML per year), Southern Rural Water in Victoria, DWER 5C licences in WA and DRDMW in Queensland. Modelling typically costs $15,000 to $80,000, and dewatering on a multi-level basement could run $150,000 to $2M or more.

Slope stability assessments typically follow the AGS 2007 Landslide Risk Management framework. These are typically triggered above roughly 15 to 20% gradient or in mapped Landslide Risk Areas. Cost is typically $8,000 to $40,000, with mitigation (soil nails, anchored retaining) running $500 to $2,500 per m² of treated face.

Mine subsidence approvals from Subsidence Advisory NSW typically cost $2,000 to $15,000 for engineering assessment, with design premiums typically 3 to 10% of superstructure cost. Ipswich, Bundaberg and Blackwater coalfields in QLD have equivalent overlays.

Rock assessments matter where rock is close to surface, such as Sydney’s Eastern Suburbs, Inner West and Lower North Shore, where sandstone often sits within 0.5 to 1.5 metres of surface. Rock hammering typically carries a two to four times premium over soil excavation ($150 to $250 per m³ vs $50 to $80 per m³), rising to $300 to $600 per m³ for sawcut or chemical splitting in neighbour-sensitive zones, plus $800 to $2,500 per rock anchor.

Dilapidation surveys are typically mandated by many councils for works within 25 to 50 metres of neighbours involving excavation over one metre, demolition, piling or vibration. They typically cost $400 to $1,500 per neighbouring property. The Mascot Towers case (June 2019 evacuation of 132 units, linked by engineer findings to dewatering at neighbouring Peak Towers causing soil fines migration) is the definitive Australian cautionary tale of dilapidation and dewatering interaction failure.

The recommended sequencing could be understood as:

1. Phase 1 ESA and title/planning review in the first fortnight
2. Preliminary geotechnical and ASS screening (co-located with Phase 2 if triggered) in weeks two to six
3. Hydrogeological, slope and salinity specialist studies in weeks four to ten
4. Dilapidation surveys once scope is defined pre-DA
5. Remediation Action Plan and Site Auditor engagement before DA lodgement

Red flags that kill feasibility

A geotechnical report could kill a deal, and developers who can read the early warning signs tend to preserve optionality. The following table summarises the typical cost impact of each red flag:

Public case studies developers could benefit from knowing include the Opal Tower defects in 2018 (which led to the NSW Design and Building Practitioners Act 2020), Mascot Towers in 2019 (multi-party settlements), the sustained geotechnical challenges of Cross River Rail (managing Brisbane Tuff, Neranleigh-Fernvale beds and Quaternary alluvium across roughly 50 boreholes including 66 metres at Kangaroo Point), Homebush Bay dioxin remediation, Fishermans Bend redevelopment, and Elizabeth Quay ASS dredging.

Commissioning the right consultant

It may be worth verifying CPEng, NER and, where required, RPEQ, RPEV or NSW DBP registration numbers on the public registers. Requesting a current Professional Indemnity certificate of currency naming insurer, limit and retroactive date is typically standard practice.

PI levels could typically be at least:

• $1M to $2M for a single-dwelling site classifier
• $5M to $10M for mid-sized firms on commercial work
• $10M to $20M for major projects

A mid-sized geotech firm with around $6M revenue typically pays $120,000 to $200,000 per year in PI premiums, reflecting that insurers price geotech alongside structural as high-risk.

A good brief typically specifies:

• Site details and survey or topo plan
• BYDA services clearance
• Project description with basement depth and approximate column loads
• Regulatory objectives (AS 1726:2017, AS 2870, AS 3798, AS 2159 as relevant)
• Deliverables (factual and interpretive)
• Field program with named borehole or CPT numbers and depths
• NATA laboratory testing scope
• Named RPEQ or CPEng signatory
• PI cover, timeline and fee basis

Red flags in consultant selection typically include a quote so cheap that it implies hand-auger site classification where a DSI is required, no PI certificate or cover under $2M for anything beyond a house, a non-engineer signatory, no RPEQ or CPEng name, refusal to issue factual logs or reliance letters, and a refusal to personally attend site.

Peer review may be warranted where proposed remediation materially affects feasibility (over $200,000 or 10% of build cost), where assumptions appear conservative, or before litigation. Typical peer review fees are $5,000 to $25,000, or 10 to 20% of the original fee.

The key Australian industry bodies typically include the Australian Geomechanics Society (affiliated with ISSMGE, ISRM and IAEG), Engineers Australia, Consult Australia and state regulators (BPEQ, VBA, Building Commission NSW and QBCC).

Legal and liability landscape

Geotechnical engineers in Australia typically owe a contractual duty to the client (ACL section 60 and common law), a tortious duty in negligence to the client and, in limited circumstances, to third parties. In NSW, a statutory duty under section 37 of the Design and Building Practitioners Act 2020 is owed to current and subsequent owners to avoid economic loss from defects. The DBPA duty is typically non-delegable per section 39 and retrospective for ten years.

Pafburn Pty Ltd v Owners - Strata Plan 84674 [2024] HCA 49 held that head contractors and developers may not apportion this liability to subcontractors, including geotechnical consultants, via proportionate liability.

Third-party reliance typically requires a formal reliance letter. Without one, a third party generally has no contractual or tortious claim. A proper reliance letter typically:

• Identifies the beneficiary by legal name and ACN
• Lists specific report references
• Confirms reasonable skill and care
• Confirms current PI with limit and insurer
• Is executed as a deed to create privity
• Carries a negotiated liability cap

Reliance letters typically cost $500 to $5,000 for residential, or 5 to 15% of original fee for complex commercial. It may be worth negotiating these up-front at commissioning.

The foundational Australian cases typically include:

• Bryan v Maloney (1995) 182 CLR 609 (HCA found a builder owed a tortious duty to a subsequent owner of a residential house with inadequate footings)
• Woolcock Street Investments Pty Ltd v CDG Pty Ltd (2004) 216 CLR 515 (HCA refused to extend Bryan to commercial premises; the owner had been asked to pay for geotechnical investigations and refused, and the subsequent purchaser was not “vulnerable” because it could have contracted for its own report)
• Pafburn [2024] HCA 49 on apportionment

Proportionate liability typically varies by state. NSW, Tasmania and WA permit contracting out of Part 4 CLA proportionality, but VIC and QLD do not. Statutory warranties under the NSW Home Building Act 1989 and the VIC Domestic Building Contracts Act 1995 may mean a defective geotechnical report causing foundation failure typically enlivens claims against both builder and geotech.

Geotechnical reports and feasibility

Commissioning geotechnical work typically happens at three classic points. Pre-offer desktop (cost $1,500 to $5,000, one to two weeks) could identify deal-breakers before exchange. Due diligence intrusive investigation (typically 30 to 60 days for development sites, sometimes 90 to 120) is where the real work happens. Post-settlement design-phase investigation is the lowest-leverage option because any bad findings are now the buyer’s problem.

Development-site special conditions typically give the buyer absolute (not reasonable) satisfaction discretion, include specific geotechnical clauses with clear triggers, grant vendor-supported access with make-good obligations, provide extension rights on notice, and avoid “deemed satisfaction” traps. Findings that could justify price renegotiation typically include:

• Shallow rock (Sydney $300 to $900 per m³ excavation)
• Deeper piling (screw piles $150 to $400 per metre, bored piers $800 to $2,000 per metre to rock)
• Contamination ($50 to $250 per m³ remediation)
• ASS, dewatering, shoring (secant $1,500 to $3,000 per m², soldier pile $600 to $1,200 per m²)
• Landslide risk

A demonstrable $400k to $1M-plus extra cost could typically see a vendor concede 50 to 75% on price to avoid re-marketing risk.

In the feasibility model, it may be worth carrying a geotechnical risk line as piling and substructure cost plus a difficult-ground allowance. Contingency could typically be:

• 10 to 15% at concept
• 5 to 8% post-DSI
• 3 to 5% post-CC

This should typically be separated from market and procurement contingency. On highly variable sites (sandstone-shale interfaces, old fill, karstic limestone, Melbourne inner alluvium), it may be worth stress-testing IRR on P90 costs. A three-month delay from unexpected ground on a $30M project at 7% per annum finance could add around $525,000 in interest alone. On high-geotech-risk sites, a minimum 200 to 400 basis points IRR hurdle uplift over “clean” sites could be appropriate.

With Feasly’s feasibility software, you can model geotechnical risk scenarios across multiple sites simultaneously, helping you identify which acquisitions warrant deeper investigation before offer.

Common questions developers and homeowners ask

How long does a geotechnical report take? A residential site classification is typically three to seven business days in metro areas. A DSI for a townhouse or apartment generally runs four to eight weeks. Complex sites (deep basements, landslip, contamination) could run eight to twelve weeks or more, often in phases.

How long is a report valid? No standard sets a fixed validity. Convention is typically two to three years, extending to five if nothing material has changed. The report may need to be re-validated if the development changes, if the site changes (fill, excavation, tree removal or planting, flood), or if referenced standards are superseded. Many councils treat reports over two years old as requiring a re-inspection letter.

Can I use someone else’s geotechnical report? Typically only with a reliance letter or formal assignment. Per Woolcock, a commercial purchaser without its own investigation or reliance is generally “not vulnerable” and has no tortious claim.

Do I need one for a small extension? Usually yes. AS 2870 typically requires site classification for any footing design, most councils and private certifiers require it, and the structural engineer needs soil parameters. Budget typically $500 to $1,200.

What if council does not require one? It could be worth getting one anyway. The statutory warranty risk under HBA and DBCA typically sits with the builder regardless, HBCF and DBI insurers may require it, and an $800 report could be considered cheap insurance against $50,000 to $500,000 foundation rectification.

Can I skip it to save money? Woolcock is instructive. The original owner refused to pay for geotechnical investigations, foundation problems emerged years later, and the subsequent purchaser had no successful claim. Certifiers and lenders typically refuse to proceed without one.

What is the difference between a soil test and a geotech report? A soil or site classification is AS 2870 residential only, typically $500 to $1,200 for one or two hand-augers to 1.5 metres. A geotechnical investigation is AS 1726:2017, prepared by an RPEQ or CPEng geotechnical engineer, with multiple deep boreholes or CPTs, laboratory testing, and full design advice. These typically cost $3,000 to $15,000 for residential multi-unit, and $15,000 to $100,000 or more for commercial and infrastructure.

Who pays? Convention is typically that the purchaser pays for their own DD geotech on a development site. Sophisticated vendors may commission their own DSI for the data room to de-risk the sale and charge purchasers for reliance ($1,000 to $5,000).

Can the report be used for insurance claims later? Yes. It is typically a contemporaneous record and pivotal evidence in HBA, DBCA, QBCC and DBI claims. It may be worth keeping the report, raw data and correspondence for the full ten-year long-stop.

What if the report misses something? If within scope, it is typically a professional negligence claim against the engineer, capped at PI and any contractual cap. If outside scope, disclaimers typically protect the engineer. Remedy may include independent peer review, PI notification, and proceedings under contract or DBPA section 37 (NSW) or the Wrongs Act (VIC).

Can a builder waive recommendations? Typically not without written sign-off from the report author or equivalently qualified engineer and the principal certifier. Deviation may breach AS 2870 and NCC compliance, HBA warranties, and DBPA section 37, and could void HBCF or DBI cover.

How do I know if I need a full investigation vs a site classification? Going full DSI typically makes sense for anything beyond a single detached house, including:

• Basements over 1.5 metres
• Sloping sites over 1-in-10 or in slope overlays
• Filled land or former industrial sites
• Soft or alluvial soils
• ASS or salinity zones
• Mine subsidence districts
• Known reactive clays
• Commercial and high-load structures
• Anywhere Class P is likely

A site classification may be adequate only for a standard single dwelling on flat, undisturbed residential land.

Conclusion: the economics of knowing the ground

The Australian geotechnical report could be understood as an option on information rather than a cost. A desktop study at $2,500 could buy a pre-offer decision to proceed, renegotiate or walk. A DSI at $15,000 to $45,000 typically buys accurate feasibility numbers for basement design and a defensible foundation for DA conditions. A peer-reviewed interpretive report at $60,000 may buy the lender certainty and the builder a priced fixed-price contract. Each of these spends is typically tiny relative to the feasibility swings they protect. A Class E reactive-clay rediagnosis, a 30-metre Coode Island Silt find, an acid-sulfate-soil trigger, or a landslide risk map entry could each move project economics by six or seven figures.

Three patterns typically kill projects. The first is buying the cheapest option. A $600 site classification commissioned when the site demands an $8,000 DSI typically produces a report whose scope excludes the issues that eventually cause loss. The second is commissioning too late, post-settlement, when the cost of adverse findings has nowhere to go but into the developer’s margin. The third is treating the geotechnical report as a compliance artefact rather than a feasibility input, cycling it straight to the structural engineer and never reading the executive summary or the limitations section.

The developers who tend to win on geotechnical risk typically do three things differently. They commission early (desktop pre-offer, intrusive during DD) and use findings as negotiation leverage. They brief the consultant precisely to the project, including named risk items (ASS, salinity, landslip, contamination, mine subsidence, UXO) that silence would otherwise exclude. And they read reports with a feasibility lens, translating soil class and bearing capacity directly into construction cost, programme and IRR.

In an era of tight development margins, rising NCC obligations, the Design and Building Practitioners Act and post-Pafburn liability exposure, geotechnical literacy is typically no longer a technical specialism. It may be core developer competence.