Wednesday's incident was the second evacuation within a year after explosive liquids were dumped in the shire's sewers.

At about 4pm, Eurobodalla Shire workers were routinely checking Narooma's sewage pumping stations when they detected an "explosive" level in the system.

"We found fuel which was potentially dumped in the sewer system, so we called Triple 000," a council spokesman said.

READ MORE: Hazmat crew investigates gas at Narooma Flat pumping station

Batemans Bay and Narooma Fire and Rescue Hazmat crews rushed to the pumping station at the caravan park, near the Narooma Bridge.

Approximately 100 people were evacuated from the park and a large section was blocked-off.

Crews worked to vent the dangerous fumes, with Narooma Police and NSW Ambulance on standby.

"It is virtually impossible to find out where it [the fuel] could have come from," the council spokesman said.

"Narooma has about 20 pumping stations which transfer sewage to the treatment works. There are entry points to the sewer system in everyone's yard, which run into the main lines.

"Many of the lines are connected and that means there is potential for explosion, pollution and a detrimental effect on the treatment works and biological processes. It's vital that people do the right thing. Please don't pour flammable liquids or chemicals, including fuels, down your sinks, toilets, storm-water drains, even on the ground or in the garbage - just don't do it."

Chemicals should be stored in an airtight container until they can be safely disposed of, either at a council household chemical clean up, or depending on the chemical, at waste management facilities. The council's website has information about safe disposal, or if in doubt, call the council waste staff on 4474 1024.

In October 2018, Narooma Library and a childcare centre were evacuated after fuel was detected in the sewer.