WA residents on alert as bushfires and heatwaves challenge fire crews

The Department of Fire and Emergency Service (DFES) issued an emergency warning on Wednesday night for residents in parts of the cities of Wanneroo and Swan.

Residents were urged to keep up to date with the latest emergency warnings and advice on the Emergency WA website.

A large air tanker that was flown in from NSW on Tuesday has been used to fight the flames.

An emergency warning was in place overnight for parts of Ashby, Banksia Grove, Jandabup, Mariginiup, Melaleuca, Sinagra and Tapping.

"You are in danger and need to act immediately to survive," authorities warned.

"There is a threat to lives and homes."

Anyone in the warning area east of Pinjar Road was warned it was too late to leave.

Authorities said the cause of the fire, which started at 1pm (4pm AEDT), has not been determined.

On Wednesday afternoon, a major freeway in Perth's southern suburbs re-opened after emergency services gained the upper hand on a bushfire in the area.

The Kwinana Freeway was closed in both directions and traffic was banked up for kilometres after a fire started near Rowley Road in Aubin Grove.

Hundreds of students were evacuated from Hammond Park Public School as a fast-moving blaze inched closer to the grounds.

Children at a nearby childcare centre were also sent home early due to the fire.

A severe heatwave is affecting the state's central west, lower west and south-west areas, including the Perth metropolitan area.

The Bureau of Meteorology said the heatwave was expected to with the peak of the heatwave expected to reach its peak on Thursday.

"Although there is no immediate danger you need to be aware and prepare now for the heat," Emergency WA said earlier on Wednesday.