An alert for out-of-control bushfires southeast of Perth has been downgraded amid easing conditions but authorities warn lives and homes could still be at risk.

An emergency warning for parts of Serpentine, Jarrahdale and Keysbrook was downgraded to a watch and act early on Monday as fire activity decreased.

The alert is in place for people bound by the South Western Highway to the west, Falls Road and Sheoak Road to the north, the southern point of the Serpentine Reservoir to the south and Albany Highway to the east, including Mount Cooke.

There is still a possible threat to homes within the area and conditions are changing, Emergency WA said.

The Serpentine blaze started during lightning storms near the Serpentine Dam on Saturday, about 55km south-southeast of Perth, and as of Sunday was estimated to cover 2000 to 3000 hectares of land.

A Bushfire Emergency Warning previously in place for parts of North Dandalup, Myara and Whittaker in the Shire of Murray where 80 hectares have been burnt, was downgraded to watch and act on Sunday.

More than 50 firefighters from the Bush Fire Service, Parks and Wildlife Service, and Volunteer Fire and Rescue Service have been working to bring that fire, also started by lightning, under control.

It is contained but not yet controlled.

Temperatures in Perth peaked at 35C on Saturday and reached a high of 26C on Sunday.

High fire danger ratings will remain in place for 21 districts across WA on Monday.