Fire crews in Western Australia are bracing for another day of intense conditions as they work to control a bushfire threatening lives and homes in the state's South West.

The fire has already burnt through 29,000 hectares after it was started by a lightning strike at Lane Pool Reserve, near Dwellingup, early on Wednesday.

Key points: Bushfire emergency for towns of Waroona, Preston Beach, Harvey, Wagerup, Yarloop and Cookernup

Bushfire emergency for towns of Waroona, Preston Beach, Harvey, Wagerup, Yarloop and Cookernup Up-to-date alerts on the DFES website and emergency information 133 337

Up-to-date alerts on the DFES website and emergency information 133 337 More than 100 people at evacuation centre in Pinjarra

More than 100 people at evacuation centre in Pinjarra The bushfire created its own weather system, BoM said

A bushfire emergency warning has been issued by the Department of Fire and Emergency Services (DFES) to residents from Waroona, Preston Beach, Harvey, Wagerup, Yarloop and Cookernup.

DFES said their priority now was to protect Yarloop, where the fire is passing through, and to stop the fire travelling further south.

The Southbound music festival, expected to draw crowds of up to 20,000 people from Friday to Sunday, has been cancelled due to lack of access to the festival site, with major highways closed.

The Coalfields and Albany Highways are congested, and fuel supplies at petrol stations are running low as motorists are diverted around the fires.

South West Superintendent Ricky Curtis addressed more than 200 people at a community meeting on Thursday night at the Leschenault Leisure Centre, which is now the evacuation point for the town of Harvey and those evacuated from Waroona.

He said a wind shift early on Friday would make it difficult for firefighters to try and contain the blaze.

"In the early hours of [Friday] morning we are expecting the winds to shift to the north-east and what that will do is put pressure on the South West containment lines," he said.

"We're expecting mid 30s, from a temperature perspective, and as the wind shifts to the north-east in the early hours of the morning, we'll be expecting 40 to 60 kilometre per hour winds, so not ideal.

Hundreds of people attend a bushfire briefing at the Leschenault Leisure Centre. ( ABC News: Gian De Poloni )

"We are not going to fix this problem overnight. We have 36,000 hectares of involved land, 115 kilometres of perimeter, transitioning from different fuel sources from state forest in the east to that coastal plain in the west; it is a very dynamic fire.

"We have unconfirmed reports of sheds and outbuildings and other types of assets that have been lost. To date, and this is the most important factor ... no lives have been lost."

Unconfirmed reports said an unoccupied house has been lost to the blaze.

Bureau of Meteorology spokesman Neil Bennett said the forecast was for increased winds and thunderstorms with lightning expected in the area overnight.

"The forecast for Friday at the moment is almost a carbon copy of [Thursday], with easterly winds overnight. They will be picking up in strength and then we'll see them gusty in the morning first thing, slowly easing as we go through the morning," he said.

"This is a very, very difficult fire to predict exactly what's going to be happening.

"The strengthening winds as well will help to fan those fires, so really, really tricky conditions at the moment."

Mr Bennett said the bushfire created its own weather system.

"During the course of the afternoon we saw quite a spectacular development on the fire front itself. A cloud developed within the fire zone. That cloud built to such an extent that it created its own thunderstorm," he said.

"It generated lightning in the area all formed by the area itself, so it's generating its own weather system. To see an actual thunderstorm developing in a fire situation like this isn't something that happens all that often.

"The results of that are quite spectacular, it can be seen from space. The system itself is huge."

Residents told could be up to five days before going home

Sorry, this video has expired Aerial footage shows the bushfire burning at Waroona

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 return to 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 were earlier evacuated by boat.

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

An Alcoa-owned refinery was under threat earlier, but firefighters managed to save the infrastructure from the blaze.

A firefighter in his 20's was taken to Peel Health Campus after suffering burns to his upper torso, but was later released.

Patients were also been evacuated from Harvey Hospital as the town remains under threat.

Western Power said electricity had been cut to 7,000 homes, amid unconfirmed reports hundreds of power poles had been destroyed.