A BUSHFIRE that was threatening homes in southwest WA has been contained but firefighters warn it's not yet under control.

The fire burned out 40 hectares in the southern part of Ambergate, in the City of Busselton, with authorities saying the cause of the blaze was suspicious.

DFES says the fire is contained but there is the potential for it to jump containment lines and impact nearby properties.

In a separate blaze, a Watch and Act alert remains in place for people in Miaboolya Road, Bibbawarra Road and North River Road and surrounding areas in the north eastern part of Carnarvon where the fire is producing a lot of smoke.

The slowly moving bushfire is heading in a northeasterly direction along Miaboolya Road and is out of control and unpredictable with embers likely to be blown around sparking spot fires ahead.

Heavy machinery is being used to install mineral earth breaks in order to contain the fire burning about 900km north of Perth.

DFES said the fire, which started yesterday morning and has burned out 1000 hectares, could affect plantations along North River Road.

Residents have been advised to put their bushfire survival plan into action.

The cause of the blaze is unknown.