Mystery pong sparks evacuations

A mystery odour across parts of Sydney's southwest prompted evacuations of a school and courthouse but firefighters say it's harmless.

The courthouse at Liverpool along with Evans High School and a childcare centre in Blacktown were evacuated this morning.

Fire & Rescue NSW Superintendent Ian Krimmer said crews had responded to many call-outs about the odour but air sampling had determined it was harmless, with no risk to health or risk of fire or explosion.

The school and courthouse were open again after being evacuated about 8.45am (AEDT).

Krimmer said it was first believed the odour came from bitumen laying in the Moorebank area overnight but it may in fact be coming from algal blooms dying off on the Georges River.

"It's still a bit of a mystery. We are still to determine the exact source of the odours."