Residents of Harvey are being urged to evacuate immediately as a wind change forces flames south towards the small South West town.

A bushfire emergency warning was extended on Thursday to cover Harvey, 140 kilometres south of Perth, after it was earlier issued for the nearby towns of Waroona and Preston Beach, where many residents have been forced to flee from the fast-moving blaze.

People in Harvey and surrounding areas are being advised by the Department of Fire and Emergency Services (DFES) to leave in a westerly direction along Uduc and Forestry roads, then south along Forrest Highway.

Meanwhile, more than 100 Waroona residents in an evacuation centre in Pinjarra have been told to find emergency accommodation in town, as it will be three to five days before they can returnto their homes.

DFES Incident Controller Greg Mair said the South Western Highway could be closed for a few weeks because a wooden bridge at Samson Brook had sustained severe damage.

Mr Mair said the Forrest Highway, the other main route south, also remained closed.

Preston Beach remains cut off, but residents are being evacuated by boat.

Mr Mair said people in Harvey are being urged to leave, but there would be no forced evacuations.

The warning also applies to the Alcoa mine site and nearby properties.

Mr Mair said the Alcoa refinery was under threat and being actively defended.

In a statement, Alcoa said the fire has moved to the west of its Wagerup refinery and all efforts are being made to protect infrastructure.

The company said its Willowdale mine remains completely evacuated and will