The fire is threatening lives and homes in Collie, about 200 kilometres southeast of Perth.

Authorities say the blaze, which has already burned through more than 8500 hectares, is heading east and moving fast.

A separate emergency was downgraded for a blaze which had been threatening homes in Woodridge, Seatrees and Breakwater Estates, north of Perth, for six days.

The fire is stationary and contained, allowing some evacuated residents to return home.

"It's been a long time coming," Department of Fire and Emergency Services assistant commissioner Brad Stringer said.

"We've had really unusual fluctuating breezes - that has just not helped us over the last few days.

"But if we still go through to the end of business today and early tomorrow, I'll be quite happy that we can actually get communities back where they need to be."

The Woodridge fire has destroyed almost 13,000 hectares and is being battled by more than 300 firefighters, down from a peak of 400.

They have worked in record-breaking heatwave conditions to save more than 6000 homes, with Perth sweltering through three consecutive days above 40C on Friday, Saturday and Sunday.

Temperatures have since eased but there is still a risk of flare-ups as winds strengthen.

One house was razed within the first few hours of the fire and a petrol station was also destroyed.

Western Power said more than 150 power poles and 10 kilometres of powerlines have been wiped out, and more than 250 properties are without power.

The utility has sent more than 70 workers to reconstruct the network.

Evacuation centres have been set up at Gumblossom Community Centre in Quinns Rocks and at Granville Civic Centre in Gingin.

Four schools in the area were closed but are expected to re-open on Tuesday.

A "watch and act" alert is in place for some surrounding areas including Guilderton.

Australian Associated Press