An emergency warning was put in place late on Saturday as six separate fires posed a threat to lives and properties, with one blaze at Mt Duckworth coming within three kilometres of the local township.

Heavy rain from an intense low-pressure system allowed fire crews to get the upper hand on Sunday, with the threat level reduced to a bushfire advice.

WA Police said overnight fires in Yallingup and Injinup appear deliberate and are being investigated by the arson squad and Busselton detectives.

If they were deliberately lit, Mr McGowan said, it was the work of an "appalling human being".

"And they'll have the book thrown at them when they're caught," he said.

"If someone has deliberately done it, well then that is just a disgusting, deplorable and disgraceful act.

"No doubt the arson squad will hunt them down and they'll go to jail."

The situation came after a week-long blaze north of Perth destroyed at least 86 homes and burnt through about 11,000 hectares.

Rain also helped crews bring that fire under control with Emergency WA downgrading the warning to a bushfire advice.

About 300 properties were still without power on Sunday after the fire caused significant damage to electricity infrastructure.

Almost 150 firefighters remained on the scene, mopping up and strengthening containment lines.

The cause of the fire was unknown.

Deputy incident controller Greg Mair said the blaze was now officially considered contained and controlled.

He said a number of roads across the region had been reopened and some locals were being granted limited access to their properties.

Australian Associated Press