A bushfire that triggered an emergency warning across Perth's south-eastern suburbs has been downgraded after almost 200 firefighters worked through the night to try to stop it heading towards more densely populated areas.

Key points: The fire was moving fast in north-east direction, fanned by strong winds

The fire was moving fast in north-east direction, fanned by strong winds The Kwinana Freeway is located near the fire zone but remains open

The Kwinana Freeway is located near the fire zone but remains open The worst of the fire is at watch-and-act level, according to authorities

The bushfire has destroyed one shed and flames came right up to the edge of several buildings. It has burned through more than 1,200 hectares after starting near the Kwinana Freeway in Baldivis around midday yesterday.

A large semi-rural area under a watch-and-act alert is bounded by the Kwinana Freeway to the west, Mortimer Road and Gossage Road to the north, Kargotich Road to the east and Karnup Road to the south.

Warnings are in place across the suburbs of Mundijong, Hopeland, Oldbury, Mardella, Serpentine, Baldivis, Wellard and Cardup, but the freeway remains open and is not covered by any warning.

Overnight the Department of Fire and Emergency Services (DFES) downgraded the alert from emergency level.

The DFES said there was a possible threat to lives and homes and people should leave immediately if their plan was to evacuate, or make final preparations to actively defend their homes if that was their plan.

The bushfire is contained and stationary but remains out of control.

Rural properties in Baldivis have been impacted by fire. ( ABC News )

A temporary evacuation centre has been set up at the Mike Barnett Sports Complex on the corner of Dixon Road and Goddard Street in Rockingham.

The fire began in scrubland east of the Kwinana Freeway, near the Karnup Road interchange, in Baldivis.

Fears for southern metro area

DFES Superintendent Ray Buchan said the fast-moving fire had been travelling about 2.5 kilometres per hour and there was spotting 300 to 400 metres ahead of the fire.

The fire is creating a lot of smoke in the Baldivis area south of Perth. ( ABC News: Briana Shepherd )

"There was significant spotting ahead of the fire and so the crews were effectively chasing the fire and just trying to get in front as the fire was coming into properties," he said.

Crews would work through the night to try to stop a worst-case scenario of the fire reaching the southern metropolitan areas of Byford and Southern River.

"We're working hard to try and stop the fire going into the Byford townsite, which is by no means at the moment under threat, but that's where it's potentially heading," he said.

Strong winds which fanned the bushfire yesterday are expected to drop further early on Friday morning.

Alert zone extends across south-east Perth

The total area on alert covers a huge area of Perth's south-east, expanding almost 25 kilometres from the ignition point in Baldivis to as far as Byford.

The fire is moving fast in a north-easterly direction fanned by strong winds. ( ABC News )

It is burning through paddocks and rural properties, including a horse riding school where several panicked horses could be seen in helicopter footage.

It has also jumped the Serpentine River.

A total of 180 firefighters, 60 fire trucks, three fixed-wing water bombers and five helicopters are fighting the fire, including the large Georgia Peach air crane leased from the US.

A crowd watches as fire crews tackle the bushfire burning in Baldivis. ( ABC News: Briana Shepherd )

Local resident Ray Wilson said strong winds were fanning the blaze.

"We have a lot of dry grass in the area," he said.

"With this wind and the fuel that we've got here, if you have a look around there a lot of growth in this area."