Police hunt for Perth's ash-cloud arsonist

Police suspect an arsonist is responsible for the bushfire which blanketed Perth in a large cloud of ash as it burnt through thousands of hectares in Mundaring.

Police have sought the public's help in tracking down a suspected arsonist believed to have started a bushfire that blanketed Perth in a huge cloud of ash.

Firefighters on Monday continued to mop up after the fire, with roads being reopened.

An arsonist is suspected to have started the fire in Sawyer's Valley at about 8:30am on Sunday.

Arson squad detectives have asked for the public's help to track down the driver of a grey early 2000s model Ford Falcon, which was seen on Gorrie Road in the area of Hancock Brook in the Mundaring State Forest.

Around 200 firefighters battled the blaze at its peak as it burned through about 3000 hectares of bushland and threatened homes and property near Mundaring Weir.

The fire has been contained but an emergency advice remained in place on Monday for residents about 10km south of the Great Eastern Highway near the Mundaring Weir Village and also near Mundaring Weir Road.