Authorities warned people in part of Bullsbrook in the City of Swan, about 35km from the Western Australia capital, that they could be at risk and issued a hazardous and toxic materials alert.
"During bushfire suppression operations to protect the Bullsbrook College and surrounding properties (on Wednesday, helicopters) drew from water sources that have been identified as sewage wastewater holding ponds," the Department of Fire and Emergency Services said on Thursday.
A bushfire which threatened lives and homes near the RAAF base Pearce continues to burn. (HANDOUT/DFES)
Authorities urged residents not to be alarmed and said the forecast high temperatures in the area into the weekend would help neutralise any residual bacteria contained in the effluent.
The temperature in Perth on Thursday is forecast to reach 39C followed by 41C on Friday and Saturday, and 38C expected on Sunday.
"No buildings were directly targeted during bushfire suppression activities but properties surrounding the bushfire area may have been impacted by some aircraft drift spray," the alert said.
Premier Roger Cook said an investigation would be launched into the incident.
"On this particular instance (the helitankers) accidentally drew water from a holding pond in a wastewater treatment facility," he told reporters.
"Our priority is to keep people safe and to get the fires out and in an emergency situation sometimes it doesn't always go to plan."
He said the effluent was dropped on bushland and drift spray may have landed on homes and properties.
"We are just responding from the abundance of caution," he said.
The bushfire, which has threatened lives and homes near Royal Australian Air Force base Pearce, continues to burn but it has been contained and controlled.
It reached an emergency-level warning on Wednesday and burned through nine hectares of properties and bushland before firefighters and water-bombing aircraft got the upper hand.
The Department of Defence confirmed the ponds the sewage wastewater was drawn from are fed from reticulated water supply and do not have any PFAS chemicals present. Â
Residents in the affected area who have water tanks for household use have been urged to empty their tanks and hose down structures and vehicles if they have been contaminated.
Evaporative air conditioner water reservoirs should also be emptied and their cooling pads washed.
People with vegetable gardens or fruit trees have been told not to harvest crops for two days and thoroughly wash any produce before consumption after this period.
It comes as another bushfire threatens lives and homes about 70km northeast of Albany in WA's south.
An emergency warning has been issued for Green Range, Kojaneerup South and South Stirling.
South Coast Highway is closed between Cheynes Beach Road and Borden Bremer Bay Road, along with Warriup Road.
25°C