Incident controller Peter Sutton says while the blaze is now contained the situation remains critical, with significant concerns over fire activity on the southern flank.

An emergency warning was reduced on Sunday to a watch and act message but Superintendent Sutton says the danger is not yet over, with the blaze still burning amid hot and windy conditions.

"We will be testing the containment lines on this fire. This still is quite a precarious situation," he said.

"The behaviour of the fire could change quite dramatically."

The blaze has burnt through about 300 hectares since starting in the Modong Nature Reserve on Saturday morning.

The cause is yet to be determined but authorities are "leaning towards suspicious".

About 150 firefighters remain on the scene with more crews and water-bombing aircraft on standby.

Supt Sutton said a number of buildings had been damaged including at least one house.

He said it was believed that was a derelict property, but checks were continuing with rapid damage assessment teams already on the scene.

Premier Mark McGowan praised the efforts of firefighters and urged the public to give the area a wide berth.

"Can I just urge everyone across the community to listen to the advice and I also urge people not to go and have a look," he said.

"That is disruptive and it puts you in danger, so please don't go and stickybeak."

The fire closed a number of roads in the Shire of Serpentine-Jarrahdale and the City of Kwinana, including the Kwinana Freeway between Mortimer and Rowley roads.

Western Power said electricity supplies had also been cut to some customers, with more than 200 properties currently without power.

Hot weather was forecast to continue across the region, with Perth expected to have tops in the mid to high 30s until at least Friday.

Australian Associated Press