A Preliminary Site Investigation looks at what's been on the land before you. It reviews historical aerial photographs, council records, EPA registers, dangerous-goods licences, anecdotal information, and a walk-over of the site to assess whether the land has been used for anything that might have left contamination behind. If the desktop work flags risk, a Phase 2 Detailed Site Investigation digs further — sampling soil and sometimes groundwater against the screening criteria in the National Environment Protection (Assessment of Site Contamination) Measure 1999 (commonly called the ASC NEPM, varied 2013). The ASC NEPM is given force by each state's contaminated-land legislation.

The framework matters because contamination can be a slow, expensive, and sometimes unfixable problem. A clean PSI gives a buyer confidence that the lot is suitable for residential use. An unflagged history of orchard pesticide use, a former service station, an old industrial tenancy, or imported fill of unknown origin can mean six-figure remediation costs — or, in extreme cases, an unbuildable lot.

The technical anchor is the Health Investigation Level (HIL) — the soil concentration above which further health-based assessment is warranted. The NEPM defines four land-use categories: HIL A for residential land with garden access and where children may dig (including childcare and pre-schools); HIL B for residential with minimal soil access (apartments, townhouses with paved yards); HIL C for public open space and recreational areas; HIL D for commercial and industrial. For a vacant block intended for a house with a garden, HIL A applies — and it's the strictest. Health Screening Levels (HSLs) screen specifically for vapour-intrusion risk from volatile petroleum hydrocarbons, depth- and soil-type-specific. Ecological Investigation Levels (EILs) and Ecological Screening Levels (ESLs) protect terrestrial ecosystems.

Common contaminants on Australian residential lots include lead (from historic paint dust and pre-1986 petrol fallout, especially within 100 m of busy roads), asbestos (bonded sheet fragments in fill, friable material from old industrial uses), hydrocarbons (former service stations, fuel depots, machinery yards), organochlorine and arsenic-based pesticides (former orchards, sheep dips, market gardens), heavy metals (former industrial sites, contaminated fill), and an increasingly watched group of PFAS compounds (firefighting foam — relevant where airports, defence sites, fire stations or training grounds are nearby).

Investigations are graded. A Phase 1 PSI is desktop and walk-over (typical cost $3,500–$8,000). A Phase 2 DSI is sampling and lab work ($8,000–$25,000+). Where remediation or formal regulatory clearance is needed, an accredited Site Auditor (NSW, VIC, QLD, WA and SA each have statutory auditor schemes) signs off — adding $15,000–$60,000+ for typical residential cases.

Questions worth asking the seller:

  • Has any environmental site investigation been done on this lot, and by whom?
  • What were the historical uses of this lot and its neighbours over the past 50–80 years?
  • Is the lot listed on the state contaminated-land register (EPA registers, EMR/CLR in Queensland, equivalents elsewhere)?
  • Was any imported fill used during subdivision or to level the lot, and is its source documented?
  • Are there any surface signs of concern — stained soil, dieback patches, drums, bonded sheet asbestos fragments?
  • Is the lot near a PFAS source — an airport, defence base, fire station, or fire training ground?
  • Has any historical use within ~500 m involved chemical storage or processing?

Who can help. A contaminated-land consultant — ideally with CEnvP (SC) "Site Contamination Specialist" certification, or a member of EIANZ or the Australasian Land and Groundwater Association. For statutory sign-off, an EPA-accredited Site Auditor in the relevant state. Your conveyancer pulls the contaminated-land register search as part of standard pre-contract enquiries.

Two practical points worth knowing. First, historical aerial photography is a buyer's friend. Nearmap, NSW Spatial Services, Victoria's Landata and Queensland's Globe all let you (or your consultant) trace what was on a lot decade by decade. Second, standards tighten over time — a PSI more than five years old should be refreshed before unconditional exchange, because HIL screening levels and contaminant lists have evolved (PFAS guidelines being the obvious recent example).

A clean PSI is one of the most valuable documents you can have on a vacant lot. It saves a future buyer the same investigation, and it's the foundation of any defence if a question is later raised about what's in the ground.

This article is general information only — a starting point for your own questions, not environmental, legal or engineering advice. Contamination law, screening levels and remediation standards vary by state and territory and are subject to change as regulatory science develops. Always engage a qualified contaminated-land consultant and, where relevant, an accredited Site Auditor, and request from the seller any prior environmental site investigations and the contaminated-land register search. Independent advice should be obtained before making any property decision.