Emergency warning for fire south of Perth

An emergency warning has been issued for a bushfire that is threatening properties in parts of Mundijong and surrounding areas southeast of Perth.

The blaze is heading northeast and spot fires are starting up to 130 metres ahead of the fire, the Department of Fire and Emergency Services says.

More than 100 firefighters are battling the fire and affected residents should leave immediately if the path is clear or prepare to defend their homes.

Incident controller Peter Sutton says it is already threatening homes and being driven by strong southwesterly winds.

"The fuel load in that area hasn't been burned for some time," he told ABC radio on Tuesday.

"We actually are already having properties impacted in some parts of the fire. People are actively defending their property.

"We're doing the best we can to try and stop this fire with a number of aircraft in the air. It is quite a significant incident and it will go over a few days."

Residents whose homes were under threat from a bushfire in parts of Lesmurdie, Kalamunda and Forrestfield, east of Perth, were earlier told it was safe to return home after an emergency alert was overnight downgraded to advice level.