About 100 people have been evacuated from Narooma Easts Caravan Park after an ''explosive'' gas level was found in the sewer system.
The Batemans Bay Fire and Rescue Hazmat crew has arrived at the scene and a NSW Ambulance crew is on standby.
Eurobodalla Shire Council workers were doing a routine check of the pumping station at Narooma Flat when they detected an "explosive" gas level in the sewer system, a spokesman said.
"We have found fuel which has potentially been dumped in the sewer system, so we called triple-zero to get the Hazmat crews here and now it is in their hands," the spokesman said.
He reminded the public never to pour flammable liquids or chemicals down a sewer.
"Anything can happen; there is potential for an explosion in this instance," he said.
They have pulled vents off drains to ventilate the area.
It is not known how long it will take for fumes to reach safe levels or for people to return to the park, a large section of which has been blocked off.