Acid sulfate soil in NSW: how to identify, assess, and manage it before you dig
NSW acid sulfate soil classes 1-5 explained: what triggers a management plan, how Class 1/2 blocks CDC, and what your ASSMP must cover.
Ask Chalkline about this →TL;DR
Acid sulfate soil (ASS) is naturally-occurring soil or sediment containing iron sulfides, mainly pyrite (FeS2), formed in waterlogged coastal environments. When ASS is exposed to air through excavation, drainage, or water-table lowering, the iron sulfides oxidise and generate sulfuric acid that can corrode concrete, steel, and copper plumbing, lower groundwater pH for years, and trigger fish kills if leached into waterways. On a coastal NSW site, mapped ASS class kills the CDC pathway and forces a DA with specialist assessment: Class 1 and Class 2 land is excluded from all complying development codes under SEPP (Exempt and Complying Development Codes) 2008 cl 1.19 (verified 2026-05-09). The class appears on the Section 10.7 planning certificate. Any works on Class 1 or 2 land that break the natural ground surface, or lower the water table, require development consent plus an Acid Sulfate Soils Management Plan (ASSMP) prepared in accordance with the NSW Acid Sulfate Soils Manual. Classes 3 and 4 only trigger when excavation goes deeper than 1 m or 2 m respectively. Get the 10.7 certificate before you design; if the class is flagged, engage an environmental consultant before lodging anything.
When you do this
Check the ASS class for a site:
- Before finalising the design, particularly floor level and footing depth.
- Before pricing. ASSMP preparation, soil testing, and lime treatment add cost and time.
- Before lodging a CDC application. Class 1 or 2 means you cannot use the CDC pathway at all (verified 2026-05-09, SEPP Codes 2008 cl 1.19).
- Before DA lodgement if excavation or drainage works are involved.
Who’s involved
- Builder: responsible for confirming ASS status before quoting and before work starts.
- Client (owner): typically obtains the Section 10.7 certificate as part of conveyancing or pre-purchase due diligence.
- Environmental consultant / geotechnical engineer: prepares the ASSMP and directs on-site treatment.
- Council or accredited certifier: reviews the ASSMP and applies consent conditions. Development consent must not be granted unless an ASSMP prepared in accordance with the NSW Acid Sulfate Soils Manual has been provided to the consent authority (verified 2026-05-09, NSW Standard Instrument LEP clause 6.1 model provision).
- NSW Planning Portal: source of the ASS map and Section 10.7 certificate service.
Steps
Step 1: Check the Section 10.7 certificate
The Section 10.7 planning certificate discloses whether the land is mapped as ASS Class 1 to 5. The underlying spatial data is the NSW EPI - Acid Sulfate Soils dataset, mapped at 1:25,000 across coastal NSW (verified 2026-05-09, NSW Planning Portal ASS dataset). Apply via the NSW Planning Portal before doing any design work involving excavation, drainage, or footings on a coastal site.
Step 2: Understand which class applies
The five ASS risk classes are defined by the depth at which acid sulfate material is likely to occur (verified 2026-05-09, NSW Planning Portal ASS dataset):
| Class | ASS likely location | CDC allowed? | Consent trigger |
|---|---|---|---|
| 1 | At or below the natural ground surface | No | Any works breaking ground or lowering water table |
| 2 | Below the natural ground surface | No | Any works breaking ground or lowering water table |
| 3 | Beyond 1 m below natural ground surface | Yes | Works deeper than 1 m OR lowering water table below 1 m |
| 4 | Beyond 2 m below natural ground surface | Yes | Works deeper than 2 m OR lowering water table below 2 m |
| 5 | Within 500 m of Class 1, 2, 3 or 4 land | Yes | No automatic trigger; assessment may still be prudent |
“Works” includes anything that disturbs more than one tonne of soil or lowers the water table, including footings, drainage, earthworks, and utility trenches (verified 2026-05-09, NSW Standard Instrument LEP acid sulfate soils clause model provision).
Class 1 and 2: CDC is not available. SEPP (Exempt and Complying Development Codes) 2008 cl 1.19 prohibits complying development on Class 1 or 2 land for all residential codes (Housing Code, Low Rise Housing Diversity Code, Greenfield Housing Code, Pattern Book Development Code, Rural Housing Code, Inland Code, Industrial and Business Buildings Code) (verified 2026-05-09, NSW Legislation SEPP Codes 2008). A DA is required.
Class 5: no automatic trigger, but check whether the LEP applies conditions and whether dewatering could affect adjacent Class 1 to 4 land.
Step 3: Check whether your works trigger the consent requirement
Even on Class 3 or 4 land, the trigger depends on what you are digging:
- Deep footings, basement car parks, below-ground tanks, and drainage works are the most common triggers.
- Slabs on ground on Class 3 land may not trigger if excavation stays within 1 m, but confirm with the certifier.
- Subsoil drainage that lowers the water table below the class threshold depth triggers even if excavation is shallow.
If in doubt, a preliminary site assessment by an environmental consultant ($500 to $2,000 ex-GST) is cheaper than holding costs on a stalled DA.
Step 4: Commission an Acid Sulfate Soils Assessment
Before lodging a DA on triggered land, council requires an ASSMP prepared in accordance with the NSW Acid Sulfate Soils Manual (EPA NSW, 1998). Required content (verified 2026-05-09, NSW EPA ASS Assessment Guidelines):
- Sampling plan: soil boring locations, pH and sulfide testing.
- Site investigation findings: depth to sulfidic material, acidity levels, extent.
- Treatment rationale: agricultural lime is the standard neutralising agent. Applied lime must be thoroughly mixed with excavated material. The rate is calculated to raise treated material to pH 7.0 based on actual acidity test results (not generic rates). Hydrated or slaked lime must not be used without specific approval.
- Earthworks sequencing to limit sulfidic material exposure to air; containment of acidic runoff.
- Monitoring plan: material validation before leaving site; runoff water quality.
The ASSMP must be prepared by a qualified environmental practitioner. Do not allow excavation to start until the ASSMP has been approved by council.
Step 5: Comply with consent conditions during works
Typical consent conditions on an ASS-affected DA:
- All soil disturbance supervised by the environmental consultant.
- Treated material meets validation criteria (pH target) before leaving site.
- Acidic water not discharged to stormwater, street drains, or waterways.
- Site diary and monitoring log maintained throughout.
If works uncover unexpected sulfidic material, stop and notify the consultant before continuing. Proceeding without an updated ASSMP is an offence under the Protection of the Environment Operations Act 1997 (NSW).
Other states
Queensland: ASS risk is controlled through an Acid Sulfate Soil overlay in each council’s planning scheme. Management requirements follow the Queensland Acid Sulfate Soils Technical Manual (Version 5.1, 2024), also referenced in NSW management plans (verified 2026-05-09, QLD acid sulfate soils data).
Victoria: EPA Victoria Publication 655.1 and the Victorian Coastal Acid Sulfate Soils Strategy (2009) govern coastal ASS management. Works disturbing CASS material are classified under the Industrial Waste Management Policy (Waste Acid Sulfate Soils) 1999. Check the Victorian CASS spatial dataset or ask council before digging on a low-lying coastal site (verified 2026-05-09, EPA Victoria Publication 655.1).
Documents needed
| Document | Prepared by | When |
|---|---|---|
| Section 10.7 planning certificate | Council (applied via Planning Portal) | Before design |
| Preliminary site investigation (if required) | Environmental consultant | Before DA lodgement |
| Acid Sulfate Soils Management Plan (ASSMP) | Environmental consultant | Condition of DA consent on Class 1/2; also required for triggered Class 3/4 |
| Soil sampling results and validation certificate | Environmental laboratory | During and after works |
| Monitoring log | Site supervisor / consultant | During works |
Common holds
- CDC attempted on Class 1 or 2 land: the certifier will reject it. Check the 10.7 certificate before engaging a certifier.
- ASSMP not lodged with DA: council will not issue consent without it. Treat the ASSMP as a required attachment, not a post-consent condition.
- Works started before ASSMP approval: most consent conditions make ASSMP approval a prerequisite to any ground disturbance. Jumping the gun creates an enforcement exposure.
- Lime treatment done incorrectly: using hydrated lime instead of agricultural lime, or incorrect mixing, can lead to treated material failing validation. Failed material must stay on site until re-treated. This holds the slab pour.
- Unexpected ASS depth on Class 3 or 4 site: if sulfidic material appears at a shallower depth than the class suggests, works must stop and the ASSMP updated. Build a contingency into the program for coastal sites even at lower classes.
References
- NSW Government, Environmental Planning Instrument: Acid Sulfate Soils dataset. https://www.planningportal.nsw.gov.au/opendata/dataset/epi-acid-sulfate-soils (verified 2026-05-09).
- NSW EPA, Acid Sulfate Soils Manual 1998. https://www.epa.nsw.gov.au/sites/default/files/acid-sulfate-manual-1998.pdf (verified 2026-05-09).
- NSW EPA, Acid Sulfate Soils Assessment Guidelines (Manual 2). https://www.epa.nsw.gov.au/sites/default/files/ASS-Manual-2-Assessment-Guidelines.pdf (verified 2026-05-09).
- NSW Legislation, State Environmental Planning Policy (Exempt and Complying Development Codes) 2008, cl 1.19 (Land on which complying development may not be carried out). https://legislation.nsw.gov.au/view/whole/html/inforce/current/epi-2008-0572 (verified 2026-05-09).
- NSW EPA, Acid Sulfate Soils: assessment and management. https://www.epa.nsw.gov.au/Licensing-and-Regulation/Authorised-Officers-and-Enforcement-Officers/resources-and-training/acid-sulfate-soils-assess-and-manage (verified 2026-05-09).
- EPA Victoria, Acid Sulfate Soil and Rock (Publication 655.1). https://www.epa.vic.gov.au/6551-acid-sulfate-soil-and-rock (verified 2026-05-09).
- Queensland Government, Acid Sulfate Soils series dataset. https://www.data.qld.gov.au/dataset/acid-sulfate-soils-series (verified 2026-05-09).
Related
- Acid sulfate soils (glossary): plain-English definition and class system overview
- Section 10.7 planning certificates NSW: how to read a 10.7 certificate and what each disclosure means
- DA process NSW: full DA lodgement walkthrough once the CDC pathway is blocked
- NSW CDC: step-by-step: confirms CDC eligibility checklist, including ASS class check
- Class 1a CDC eligibility checklist NSW: pre-lodgement checklist with ASS class as a line item
See also
- Pre-DA meetings: use a pre-DA meeting to clarify ASSMP scope with council before spending on investigation
- Heritage overlays: another site constraint that, like ASS, blocks CDC and forces DA assessment
- Bushfire prone area mapping: another Section 10.7 flag that affects DA and design before you start
Last updated: 2026-05-09. Verified: 2026-05-09. Quarterly review for currency.