About 85 firefighters are continuing to battle an out-of-control bushfire burning through scrub in Furnissdale near Mandurah, south of Perth.

Key points: A watch and act warning remains in place for part of Furnissdale

A watch and act warning remains in place for part of Furnissdale The bushfire was downgraded from emergency level after 5:00pm

The bushfire was downgraded from emergency level after 5:00pm The fire is moving slowly in a south-westerly direction

But an emergency warning for the fire has been downgraded to a watch and act, with homes no longer under immediate threat.

The unpredictable blaze began about 1:00pm near the intersection of Goodooga Road and Jannali Road in the southern part of Furnissdale, with aerial support sent to help firefighters.

The alert was upgraded to emergency level about 3:15pm but was downgraded two hours later as the fire moved slowly in a south-westerly direction.

Thick plumes of smoke filled the sky as the bushfire increased in intensity. ( Supplied: South Yunderup Residents and Ratepayers Association )

The watch and act warning remains in place for people on or near Goodoga Road, Paull Street, Riverside Drive and Furnissdale Road, where there is a possible threat to lives and homes.

"If you are not prepared or you plan to leave, leave now if the way is clear," the Department of Fire and Emergency Services said.

"If you are well prepared and plan to actively defend your home, make final preparations now."

The fire began about 1:00pm with an emergency warning issued around two hours later. ( ABC News )

The blaze has burnt though 30 hectares.

The area under threat lies alongside the Serpentine River, making access difficult for fire crews in some parts.

Some roads may be closed and motorists have been asked to avoid the area, reduce speed and drive carefully due to smoke.

The cause of the fire is unknown.