Fire crews are fighting a scrub fire along Safety Bay Road, near the Kwinana Freeway, in Baldivis.
It is believed there are flames on a median strip at Safety Bay Road but the main part of the fire is on the eastern side of the Freeway, reportedly about 400m north of Safety Bay Road.
The blaze is being battled by 30 career and volunteer firefighters, two helitacs and two helicopters.
The fire is contained and not controlled and is moving slowly in a westerly direction.
Police are also heading to the scene to monitor traffic after reports of smoke billowing across the freeway.
About 2ha have been burnt but there is currently no threat to lives or homes.
The cause of the fire is not known.
A bushfire advice warning has been issued for Baldivis residents between Safety Bay Road and Mundijong Road.
Kwinana Freeway is closed between Safety Bay Road and Mundijong Road.
The Department of Fire and Emergency Services has warned other roads may be closed and motorists are advised to avoid the area, reduce speed and drive carefully because of the smoke.