Unexpected Contamination on Site: Managing Risk, Compliance and Project Outcomes During Construction

Prepared by Confluence Environmental - environmental and contaminated land consultants supporting development, remediation and validation projects across NSW.

Unexpected contamination remains one of the most common causes of delay, cost escalation and regulatory intervention on construction and redevelopment sites. Even where preliminary investigations have been completed, contamination or hazardous materials are frequently discovered once ground disturbance begins.

For developers, builders and project managers, the critical issue is rarely whether contamination exists - but how it is identified, managed and documented once discovered. The response taken in the first few days often determines whether the issue is resolved efficiently or becomes a prolonged approval and compliance problem.

This article explains how unexpected contamination typically arises, what is expected from a regulatory and planning perspective in NSW, and how a structured, proportionate response can protect both people and project outcomes.

What Is Meant by “Unexpected Contamination”?

Unexpected contamination refers to contamination or hazardous materials encountered during site works that were not anticipated, or not anticipated to the extent, depth or concentration identified during earlier investigations.

In practice, this commonly occurs because subsurface conditions vary across a site, historical records are incomplete, access limitations constrained earlier investigations, or deeper excavation exposes previously undisturbed materials.

Importantly, the discovery of unexpected contamination does not automatically indicate that earlier investigations were inadequate. It reflects the inherent uncertainty associated with subsurface environments, particularly on previously developed land.

Common Types of Unexpected Findings During Construction

Unexpected contamination most commonly presents during excavation, demolition or service installation. Typical examples include:

Impacted soil or fill
Soils exhibiting staining, odour or unusual composition may indicate contamination by petroleum hydrocarbons, volatile organic compounds, heavy metals or other substances not previously identified.

Asbestos or hazardous building materials
Asbestos-containing materials are frequently discovered in fill, demolition rubble, underground services or buried structures that were not accessible during pre-works inspections.

Underground petroleum storage systems (UPSS)
Former fuel tanks, pipework or associated infrastructure may remain in situ long after surface features have been removed, particularly at older service station, industrial or transport sites.

Landfill material or leachate
Uncontrolled fill or historical waste disposal can introduce landfill material, leachate or contaminated groundwater into excavations, often at depth.

Each of these scenarios carries different health, environmental and regulatory implications and requires a site-specific response.

Why Early and Appropriate Response Is Critical

From a health and safety perspective, uncontrolled disturbance of contaminated material can expose workers and others to hazardous substances.

From an environmental perspective, continued excavation without controls can spread contamination or create off-site impacts via groundwater, surface water or vapour pathways.

From a planning and regulatory perspective in NSW, poor handling of unexpected contamination can trigger stop-work directions, activate additional development consent conditions, delay occupation or completion certificates, and increase long-term liability for landowners and developers.

Conversely, councils and regulators generally respond favourably where contamination is identified early and managed transparently and defensibly.

Immediate Actions When Unexpected Contamination Is Discovered

While responses must always be site-specific, the following steps are typically expected and appropriate:

Works in the affected area should be paused to prevent further disturbance until risks are understood. In many cases, construction can continue in unaffected areas of the site.

Interim controls should be implemented to manage immediate risk. This may include isolating the area, restricting access, covering exposed materials, dust suppression or odour management.

Key project stakeholders, including site supervisors and project managers, should be notified immediately so that an appropriate response can be coordinated.

Early engagement of an environmental consultant allows regulatory obligations to be clarified and an appropriate investigation pathway to be established. Depending on the circumstances and approval conditions, notification to council or the EPA may also be required.

Regulatory and Planning Considerations in NSW

In NSW, contaminated land is managed through a combination of planning controls, EPA guidance and work health and safety obligations.

When unexpected contamination is encountered, regulators typically expect prompt identification and control of risk, proportionate investigation to define extent and significance, clear documentation of decisions and outcomes, and alignment with development consent conditions and the intended land use.

The regulatory focus is rarely on whether contamination exists, but on whether it has been managed appropriately.

Assessment Pathway After Discovery

Once immediate risks are controlled, a staged and proportionate assessment approach is typically adopted.

This usually involves targeted investigation to define the nature and extent of contamination, followed by an update of the conceptual site model to reflect new sources, pathways and receptors.

Based on the findings, it is then determined whether additional investigation is required, such as a Preliminary Site Investigation addendum or a Detailed Site Investigation, and whether remediation or management measures are needed to support the proposed development.

This approach avoids both under-assessment and unnecessary escalation of scope.

Managing Program, Cost and Project Risk

Unexpected contamination does not automatically result in major delays. In many cases, impacts can be effectively managed by engaging technical advisors early, maintaining clear communication with council or regulators, staging investigation works to align with construction priorities, and integrating remediation with planned excavation activities.

Projects that attempt to continue works without assessment often experience greater delays than those that pause briefly and respond properly.

Illustrative Scenario

During basement excavation for a mixed-use development, stained soil and fuel odours were encountered at depth. Works were paused in the affected area and an environmental consultant was engaged.

Targeted investigation identified petroleum hydrocarbon contamination associated with a former underground petroleum storage system. The impacted area was limited and managed through controlled excavation, waste classification and off-site disposal, followed by validation sampling.

Because the issue was identified early and addressed transparently, the project proceeded with minimal impact on the overall construction program.

Key Takeaways for Developers and Project Teams

Unexpected contamination is common on previously developed land. Early identification and response reduces risk and cost, and regulators value transparency and defensible decision-making.

Briefly stopping work often avoids longer delays later, and an integrated approach to investigation, remediation and validation is the most effective way to manage risk and maintain program certainty.

Frequently Asked Questions

Do I need to notify council if contamination is found during works?
This depends on the nature of the contamination and development consent conditions. In many cases, councils expect notification where contamination is materially different from what was previously assessed.

Can construction continue while contamination is assessed?
Often yes. Works can usually continue in unaffected areas while the issue is investigated and managed.

Will unexpected contamination delay my development approval or occupation certificate?
Not necessarily. Delays typically arise where contamination is poorly managed or inadequately documented.

Who is responsible for investigation and remediation costs?
Responsibility generally sits with the landowner or developer, subject to contractual arrangements and site history.

Getting the Right Advice Early

Unexpected contamination does not need to derail a project but it does require informed, timely decision-making.

Confluence Environmental supports developers, builders and asset owners with contaminated land assessment, remediation planning and validation services across NSW. Our approach focuses on practical, proportionate solutions that align technical, regulatory and project delivery requirements.