The blaze in the Leeuwin-Naturaliste National Park has ripped through almost 5500 hectares since Wednesday.

Police say they believe someone deliberately lit the fire near Mammoth Cave, based on an initial assessment of the scene.

"Detectives would like to hear from anyone who saw any suspicious activity in the vicinity of Mammoth Cave on Wednesday (8 December) or who has dash-cam or mobile phone vision of people or vehicles in the area at the time," police said in a statement on Saturday.

On Saturday morning an evacuation warning was reactivated for people in the national park, but it was downgraded by Saturday night amid light rain and a favourable weather forecast.

The downgraded "watch and act" alert applies to people bounded by Calgardup Brook to the north, Caves Rd to the national park boundary to the north and east, Hamelin Bay Road West to the south, and the coastline to the west in the shire of Augusta-Margaret River.

Earlier, Department of Fire and Emergency Services incident controller Brad Barton said the fire was contained but not controlled.

He said there were no reports of homes lost and the blaze had been mainly contained to the national park.

"We're going to get to a tropical 19 degrees in Margaret River tomorrow (which) is the forecast with some southerly winds. We've got some light misty rain at the moment so that has really helped us," he told a livestreamed community update on Saturday afternoon.

Mr Barton said residents should still take the situation seriously and monitor the Emergency WA site for updates.

"Fire is unpredictable and ... things can go awry at times," he warned.

Seventy-five firefighters worked to fight the blaze on Saturday.

Residents had first been urged to leave on Thursday before conditions improved on Friday, but later deteriorated.

Australian Associated Press