Scorching weather fans bushfires across parts of WA

An out-of-control bushfire is threatening lives and homes on Western Australia's southern coast.

A watch and act alert is in place for the western part of Bremer Bay and the eastern part of Boxwood Hill in the Shire of Jerramungup.

People have been warned to leave or get ready to defend properties.

Reef Beach, surrounding sand dunes and nearby camping areas have been closed.

"People camping in the area need to leave immediately," the Department of Fire and Emergency Services warns.

The blaze, which was reported yesterday morning, has burnt 200 hectares so far but the cause is unknown.

Parts of the state are sizzling through a scorching heatwave, with Perth reaching a top of 37.2 degrees this afternoon.

The weather is expected to stay hot until boxing day.