Rural Fire Service (RFS) volunteers were soaked with untreated sewage while fighting a fire near Port Macquarie, on the New South Wales mid-north coast last week.

The fire-fighting helicopter mistakenly scooped water from a raw sewage pond to water-bomb a fire near Kew.

The Australian Workers Union is now calling for a full investigation into the incident, which potentially affected up to 30 RFS volunteers.

AWU spokesman Mark Hughes says State Forest crews were also affected.

"For some strange reason, around 7:30 in the morning during the water-bombing, a call was made from the RFS Commanders to use the sewage treatment plant rather than a much more accessible water sources such as the Camden Haven River, Queens Lake or the Cowarra Dam," he said.

"There are primary and secondary treatment plants, so basically you've got one pond that's got raw sewage and one that's been treated.

"What they were dumping was the untreated sewage."

Mr Hughes says those affected could face serious health implications.

"The RFS to their credit did, after the event, have the HAZMAT team there to shut down the site after they'd been water bombing with the sewage for three hours," he said.

"They did have paramedics there to talk to everybody and decontaminate on the scene.

"But there was no initial follow up from them until they were prompted."