A bushfire that threatened lives and homes near a popular hiking spot in the Perth Hills has been downgraded, while residents in the city's south are experiencing power blackouts.

An emergency warning was declared just after 4:00pm for a fire burning in Lesmurdie Falls National Park in the suburbs of Forrestfield, Kalamunda and Lesmurdie.

Helicopter footage showed flames taking hold of trees in the park and burning close to homes, about 24 kilometres east of Perth's CBD.

Although the threat has eased, firefighters say the blaze is still out of control.

The bushfire has burnt through scrub land in the Lesmurdie Falls National Park. ( ABC News )

The fire was moving through the park in a north-easterly direction.

Almost nine hectares of scrub land has been burnt.

About 100 firefighters were last night at the scene trying to control the blaze.

Lesmurdie Falls National Park is a popular hiking and sightseeing destination, about 24 kilometres east of Perth's CBD. ( ABC News: Gian De Poloni )

A watch and act alert is current for people in an area bounded by Yorna Road to the north, Orange Valley Road and Karralika Avenue to the east, Falls Road to the south and Ozone Terrace to the west in Lesmurdie and Kalamunda.

A bushfire advice alert has also been declared for areas west of the fire, including Forrestfield and Maida Vale.

It is not known at this stage how the fire began.

Power outages in Perth's south

Meanwhile, about 16,300 homes were without power in Perth's southern suburbs after a fault at a substation triggered widespread blackouts.

Bushfire burns in rocky terrain close to overhead powerlines in Lesmurdie. ( ABC News )

Homes in Kwinana, Aubin Grove and parts of Baldivis were affected, as well as semi-rural properties in Casurina, Wandi and Oldbury.

Almost a dozen traffic signals were out of order in the area.

Western Power said the network was experiencing high energy use due to hot weather.