A water-bombing helicopter has mistakenly sprayed partially treated sewage onto firefighters battling a blaze on the NSW mid-north coast.

The Rural Fire Service has launched an investigation into last Tuesday's incident but says all firefighters involved have undergone medical checks and none have shown any ill effects.

The mistaken drop took place in the Kew area, near Port Macquarie, where firefighters and four aircraft were battling a blaze.

The Australian Workers Union urged an investigation after RFS volunteers and state forest crews were affected.

An RFS spokesman said on Monday that a helicopter mistakenly drew up secondary treatment water from a sewage treatment plant.