A RURAL Fire Service investigation is under way after untreated water from a sewage plant was dumped on volunteer firefighters battling a blaze on the mid-north coast.

Up to 50 firefighters and four aircraft were working on the 30ha bushfire at Kew, near Port Macquarie, on Tuesday when a helicopter mistakenly drew up the wastewater from the wrong pond at the nearby sewage treatment plant and dropped it on them.

More than half the firefighters had to be decontaminated, along with their equipment, and were treated by paramedics at the scene.

None reported any adverse symptoms from the incident.

An RFS spokeswoman said 12 firefighters had been directly exposed to the "secondary treatment" water, while a further seven were in the general area.

"All 29 firefighters on the fireground and their equipment were immediately withdrawn and decontaminated by Fire and Rescue NSW," the spokeswoman said. "As a precaution, each firefighter has since been provided with further medical follow-up. At this time, no firefighters have complained of any ill-effects. They will continue to be monitored by the Rural Fire Service."

The fire, which began Tuesday morning, forced closures of the Pacific Hwy, but was fully under control and being patrolled by Thursday.

The wastewater dumped on the firefighters came from the Kew sewage treatment plant run by Port Macquarie-Hastings Council.

A council staff member said employees had been advised not to comment on the incident because it had been under the management of the RFS.

"We weren't in control of it whatsoever so it's not something that we'd comment on," the staff member said.

According to the Sydney Water website, most of the rubbish and 60 per cent of solid material has been filtered out of wastewater once it passes through primary treatment.Rural Fire Service Association general manager Bernard Cox said he had never heard of such an incident happening before.

"They shouldn't have taken it from (that pond) and that's why they probably will do some kind of investigation about why it was picked up," said Mr Cox, who praised the RFS for their quick thinking in protecting the firefighters.