A preliminary site investigation (PSI) uses site history and a walkover inspection to assess if land may be contaminated. A detailed site investigation (DSI) uses intrusive soil, groundwater or vapour sampling to confirm contamination and measure its extent.
In NSW, the PSI usually comes first. A DSI is initiated when the PSI identifies a potential contamination risk.
This guide compares the two investigations side by side and explains when a PSI leads to a DSI. It also walks through common development and property scenarios to help you work out which investigation your project needs.
Key takeaways
- Choose a PSI first for development applications, rezoning and property due diligence.
- Expect a DSI when the PSI finds a possible contamination source near people or the environment.
- Budget for sampling and laboratory costs at the DSI stage, not the PSI stage.
- Plan for remediation only after DSI results confirm contamination above the relevant criteria.
What is a preliminary site investigation?
A preliminary site investigation is the first stage of a contaminated land assessment. It reviews historical records, aerial photos and past land uses to identify likely contamination sources.
Most preliminary site investigations are desktop-based and rely on a walkover rather than sampling. The NSW EPA’s contaminated land reporting guidelines set out what each PSI report must include.
What is a detailed site investigation?
A detailed site investigation measures the type, concentration and spread of contaminants on a site. It builds on the PSI findings and targets the areas of concern the PSI identified.
Results are compared against health and ecological criteria for the proposed land use. The DSI report then states if the site is suitable or needs remediation.
PSI vs DSI at a glance
| Preliminary site investigation (PSI) | Detailed site investigation (DSI) | |
|---|---|---|
| Purpose | Identify potential contamination sources | Confirm and measure contamination |
| Method | Desktop review and site walkover | Intrusive sampling and laboratory analysis |
| Sampling | Limited or none | Soil, groundwater, soil vapour or surface water |
| Typical trigger | Development application or due diligence | PSI findings or council request |
| Outcome | Recommendation on further investigation | Contamination extent and remediation need |
| Relative cost | Lower | Higher |
When does a PSI lead to further investigation?
A PSI leads to a DSI when it identifies a credible link between a contamination source and people or the environment. Under section 4.6 of the Resilience and Hazards SEPP, councils can request a detailed investigation if PSI findings warrant one.
Common scenarios include:
- Former service station converted to apartments — Underground fuel tanks call for groundwater and soil vapour sampling.
- Market garden rezoned for housing — Pesticide use calls for soil testing across the former growing areas.
- Factory site bought for redevelopment — A history of chemical storage calls for testing before the purchase settles.
- Suburban house knock-down rebuild — A residential history with no industrial use often ends at the PSI stage.
Does a detailed site investigation include soil testing?
Yes. Soil testing is a core part of every DSI. Consultants collect samples from test pits, boreholes or hand augers at set depths. Groundwater and soil vapour samples are added where the PSI points to fuel, solvents or other mobile contaminants.
The NSW EPA’s sampling design guidelines set the number, layout and depth of sampling points. All samples are analysed by NATA-accredited laboratories.
Which site investigation does your project need?
Start with a PSI for most development applications, land use changes and property purchases. Move to a DSI when the PSI identifies a risk or council requests one. Some sites with a well-known contaminating history may skip straight to a DSI, though most councils still expect the PSI findings.
If contamination is confirmed, a remedial action plan follows. For excavation projects, an unexpected finds protocol for excavation works guides crews if they uncover contamination during earthworks.
Arrange a preliminary or detailed site investigation with EnviroX Consulting
EnviroX Consulting prepares preliminary and detailed site investigations for residential, commercial and infrastructure projects across Sydney and NSW. Reports follow NSW EPA guidelines and support development applications and property transactions.
Reach out to us to request a quote and discuss your site.
FAQs
What is the difference between a PSI and a DSI?
A PSI assesses contamination risk using site history and observations. A DSI confirms contamination through sampling and laboratory analysis.
Do I need a PSI before a detailed site investigation?
In most cases, yes. The PSI identifies where and what to sample, which keeps the DSI focused.
Does every PSI require soil testing?
No. Most PSIs rely on records and a walkover. Limited sampling is added only when a specific concern needs screening.
What happens if potential contamination is identified?
The consultant recommends a DSI to measure the contamination. If results exceed criteria for the proposed use, a remedial action plan follows.