A significant safety concern has emerged regarding a major dam in Sydney's Thornleigh Reservoir, which has failed three safety audits, raising fears of a potential collapse that could threaten the lives of nearly 150 people.

Internal government documents have revealed that audits conducted by engineering firm GHD identified slope displacement and leakage issues at the dam, indicating possible slope instability.

The most critical risk identified is the sudden failure of the dam wall, which poses a direct danger to residents living downstream.

In response to these alarming findings, Water Minister Rose Jackson announced that sensors have been installed to monitor the dam, and the water level is being maintained lower than usual as a precaution.

Despite these safety concerns, the NSW government maintains that the dam is safe, classifying the identified safety issues as merely 'theoretical' and asserting that there is no elevated risk for nearby residents.

Greens MP Sue Higginson has raised additional concerns, stating that a proposed road by Hornsby Council could jeopardize the dam's integrity, linking it to a contentious redevelopment project in the area.

Concerns about the road's impact on the dam were initially raised in 2019, and the situation has become more complex due to stalled negotiations over the redevelopment plans.

In addition to the potential loss of life, a dam failure could drastically reduce the water supply for 834,000 residents who depend on it.