Water bombers from New South Wales have been sent to assist weary firefighters who are battling multiple emergency fires, including a massive blaze in Perth's north which has entered its fifth day.

Key points: New South Wales has sent a Boeing 737 waterbomber to help fight bushfires in WA

New South Wales has sent a Boeing 737 waterbomber to help fight bushfires in WA Two emergency warnings are active in Yanchep and Collie, in the South West

Two emergency warnings are active in Yanchep and Collie, in the South West The Yanchep fire has burnt through almost 12,000 hectares since Wednesday

About 400 firefighters are at a fire near Two Rocks, about 60 kilometres north of Perth, that has torn through almost 12,000 hectares of land.

Bushfire warnings cover a 50-kilometre stretch of coast including the towns of Guilderton, Seabird and Woodridge.

Another emergency warning was broadcast shortly after 3:00pm on Sunday for the small country outpost of Mogumber, about 70 kilometres north-east of Two Rocks.

That fire was reported at 1:54pm, and started near the intersection of the Bindoon-Moora Road and Mogumber Road West, in Mogumber.

Meanwhile, another emergency warning is active for a fire burning near Collie in the state's south-west, about 160 kilometres south-east of Perth.

Australia's biggest firefighting aircraft, a Boeing 737 air tanker, can hold 15,000 litres of water and has been already been deployed to the Collie fire, after flying in from NSW overnight.

Australia's biggest firefighting aircraft, the Boeing 737 large air tanker, was sent to WA. ( Supplied: New South Wales Rural Fire Service )

Premier Mark McGowan said he was keen to see the plane used to battle the fires.

"It will be bombing fires around the state over the next couple of days — clearly the weather conditions are extreme and therefore it requires every effort put into dealing with a serious situation," he said.

WA's Fire and Emergency Services Commissioner Darren Klemm said the plane will stay in WA until at least Monday night.

Emergency Services Minister Fran Logan and Commissioner Darren Klemm. ( ABC News: Emily Piesse )

Commissioner Klemm said there was a high risk of further flare ups with more wind changes.

Temperatures in Perth have soared above 43 degrees Celsius in some areas. It was the first time the city had three consecutive days above 40C in December since weather records began.

"Things are reasonably benign at the moment in terms of the wind, but we're going to have difficulties again with the strength of the winds," he said.

Arson has been ruled out as the cause of the Two Rocks blaze but the Commissioner would not elaborate further, saying the investigation was ongoing.

The fires around Two Rocks in the past 72 hours. ( Source: Landgate )

He also confirmed a firefighter was injured by a falling tree overnight.

Two Rocks fire 'could burn for weeks'

Deputy incident controller for the Two Rocks fire, Superintendent Sven Anderson, warned the fire may continue burning within containment lines for "days, possibly weeks".

Superintendent Anderson said despite temperatures forecast to reach 35C tomorrow, after three days above 40C, conditions will still be challenging.

"Unfortunately it's only a slight drop," he said.

"The humidity does come up, and all those sorts of things do come in our favour, but it's still going to be hot and windy."

Authorities said road closures and warning areas for the Yanchep fire would be reviewed today. ( ABC News: James Carmody )

Fire and Emergency Services Minister Fran Logan said firefighters had managed to save 6,000 homes.

"The work that all of the firies and all of the volunteers have put in over the last few days is nothing short of remarkable," he said.

"If you look at the map of where the fires have gone, it is absolutely astonishing that we haven't lost more than one house."

One home, a petrol station and some buildings associated with Yanchep National Park were lost when the bushfire started on Wednesday.

Stephen Brown stayed to defend his and his neighbours property at Breakwater Estate.

Stephen Brown stayed and defended property in Breakwater Estate, near Two Rocks. ( ABC News: Emily Piesse )

"I think without all of the firies on the crew, there'd be nothing left in here. They did a brilliant job and I think they saved a lot of houses in here," he said.

"At one stage they were standing with their back to the fire and just hosing down the embers because they couldn't stop it."

Collie fires rage outside town

Two fires are burning near Collie, about 200km south of Perth, one south-west of Wellington Dam and the other in dense jarrah forest to the north near Yourdamung.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 28 seconds 1 m 28 s Fire north of Collie burns through large swathes of forest

The fire near Yourdamung Lake has burnt through 7,400 hectares since it began on Saturday afternoon.

Both fires were expected to remain at emergency level throughout the night.

More than 50 firefighters have been battling the out-of-control blaze.

Collie volunteer firefighter Susie McKee, resting after her second shift on the Collie fire front. ( ABC News; Anthony Pancia )

Parks and Wildlife incident controller Brad Barton said wind changes on Sunday afternoon had helped firefighters protect the Collie townsite, and would continue to blow the fire away from Collie overnight.

"It's pushing into some prescribed burned areas which are low-fuel areas, as well as an old fire scar from the lower-Hotham fire, which are really helping our containment efforts," he said.

"With the southerly aspect and southerly winds, the threat to the Collie townsite has now at least abated for the time being."

Mr Barton said the Boeing 737, which worked along the southern edge of the Yourdamung fire, had helped firefighters.

"It certainly allowed us to get our tracking along that edge a lot quicker, so it's been a great support," he said.

Residents fight back flames

Yourdamung Road retreat owner Russell Tierney said it was the first time he had stayed and defended a property under threat from fire.

"I'm feeling real stressed, but my mates are here to give us a hand so it's all good," he said.

Russell Tierney actively defended his Collie property from the threat of fire. ( ABC South West: Laura Birch )

"It was hairy before, ash started falling on my place. That's when I started calling mates up saying come around with your tankers.

"There's a lot of bush around here — this was only (backburned) last year but it's burning still now anyway.

A thick layer of smoke from the Collie fire hangs over Bunbury. ( Supplied: Rodney Peterson )

"It's dry. It's very dry out there so it will burn."

The fire has blanketed the nearby port city of Bunbury with a thick layer of smoke.

A smoke alert has been issued for Bunbury and other major South West towns. ( Supplied: Trevor Morgan )

The smoke has also impacted on other towns in WA's south-west, with a smoke alert current for Busselton, Margaret River and Donnybrook.

Wellington National Park is closed, and people already in the area are advised to leave for a safer place.

A total fire ban remains were in place across the Perth metropolitan area on Sunday.