Bushfire emergency warning south of Perth

An emergency warning remains in place for a bushfire that is threatening lives and homes across a string of suburbs in Perth's south.

About 150 firefighters are battling the blaze which was reported at midday on Thursday near Baldivis, about 40km south of Perth.

It has since burnt through more than 300 hectares in strong winds.

The blaze is moving fast in a northeasterly direction and the emergency warning applies across an area spanning 20km north to Casuarina.

Affected residents should leave immediately if there is a clear path, the Department of Fire and Emergency Services says.

A watch and act alert has been issued for surrounding suburbs east of Kwinana Freeway and authorities are warning residents as far north as Cockburn to remain vigilant.

An evacuation centre has opened at Mike Barnett Sports Complex in Rockingham.