Firefighters from Helensburgh and Shellharbour were called to Lower Coast Road just after 7am on Tuesday, November 21 following reports a substance was bubbling and heading towards Stanwell Creek.

"It appears to be a soapy type detergent," Fire and Rescue NSW Inspector Andrew Barber told the Mercury as crews arrived on scene.

"What we're concerned about is the watercourse ... the lower part of the creek near the causeway is at risk."

Specialist HAZMAT crews tested the bubbly, filmy substance and while no conclusive result was found, it was found to have a neutral pH.

FRNSW Shellharbour Station Officer Trevor Brown said a water main was opened to allow firefighters to dilute the substance and help it disperse.

Recently Shellharbour's HAZMAT crews invited the Mercury inside the station to see how they deal with toxic spills, drug labs and explosives.