Residents south of Perth have been warned it is not yet safe to return to their properties after a bushfire destroyed several buildings including three homes.

An emergency warning was reduced to a watch-and-act alert on Tuesday for communities in parts of Coolup, Meelon, Nanga Brook, Teesdale and Waroona in the shires of Waroona and Murray.

Authorities said the bushfire was contained but not controlled, with firefighters working to consolidate containment lines.

There was still a possible threat to lives and homes as conditions were changing.

The fire, which has about a 50km perimeter, started on Saturday in state forest west of the Nanga township and has burned about 3150 hectares.

The blaze is thought to have been accidentally sparked by campers trying to charge a phone with a car battery.

Fire and Emergency Services Commissioner Darren Klemm said three homes and five short-stay accommodation units had been razed.

More than 200 firefighters and support personnel are battling the fire with support from water bombers.

“The fact that there has been no loss of life is testament to the hard work of our firefighters and the strategic use of aircraft,” Klemm said.

“For those families that have lost homes, we understand this could be a difficult time for them,” he told reporters.

“We’ll be making sure we’re reaching out and providing support to them.”