A bushfire burning in Perth's south has been downgraded to a watch and act. -AAP Image
A bushfire that closed a major freeway and threatened homes in Perth's south has been downgraded, but nearby residents have been warned to remain alert.
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A watch and act alert has been issued for parts of Aubin Grove, Hammond Park and Wattleup, about 30km south of the city centre.
An emergency warning was in place earlier on Wednesday.
"There is a possible threat to lives and homes as a fire is burning in the area and the conditions are changing," the Department of Fire and Emergency Services said in the updated advice.
"If you plan to stay and actively defend, do not rely on mains water pressure as it may be affected.
 "You need to have access to an independent water supply, and start patrolling your property to put out spotfires."
The blaze started near the intersection of the busy Kwinana Freeway and Rowley Rd in Aubin Grove.
People in the area were urged to leave west along Russell Rd or north along Hammond Rd if safe to do so.
The fire initially closed the freeway but it has since reopened.
Western Australia is currently in the grips of a heatwave, with Perth forecast to reach 39C on Wednesday.
31°C