Firefighters have spotted a wild “fire devil” amid the raging bushfires in an Aussie state as millions of others brace for more stifling heat.

Residents in multiple states have been told to brace for more intense heat and bushfires after a “fire devil” was spotted among the flames in South Australia.

Despite a burst of cold weather washing over parts of the country’s southeast this week, extreme heat is returning to smash parts of Western Australia, SA, Victoria and NSW as the mercury creeps higher.

Temperatures are forecast to tip into the high 40s across central Western Australia and the 30s along the coast before spreading through South Australia, NSW and parts of Victoria.

A ‘fire devil’ has been spotted amid the bushfires in Deep Creek, SA. Picture: Morphett Vale Country Fire Service
A severe heatwave warning in WA impacts areas spanning the Gascoyne and Central West regions, including Carnarvon and Coral Bay.

The mercury is forecast to reach a sizzling 42C in Newman, 39C in Karratha and 37C between Exmouth and Geraldton.

It will also be a hot one in Perth, where temperatures areas forecast to reach 32C on Thursday, while conditions will be slightly cooler along the coast.

The heat is picking up again. Picture: Windy
As the heat increases, so does the fire threat.

Several bushfires are continuing to burn across Deep Creek on the Fleurieu Peninsula south of Adelaide. The fires have ripped through about 3400 hectares of and destroyed at least three structures.

The horror fires have been exacerbated by the extreme heat and blistering winds, which whipped up a “fire devil” in Deep Creek on Wednesday afternoon.

Wild footage captured by firefighters at the Morphett Vale Fire Service showed the whirlwind of fire and ash as it thrashed in the sky among the smoke.

The SA Country Fire Service said the Deep Creek fire would likely continue to burn erratically as strong winds encourage the flames to change direction and spread further.

“(The) community are reminded to stay out of the current warning area where possible,” a statement read.

“Anyone within the impacted area will be allowed to leave via the closed roads but will not be able to return until they are reopened.

“Given the unpredictable nature of this fire, any road closures could change at short notice.”

While conditions are forecast to hit a sweltering 41C in Roxby Downs and 40C in Marree and Coober Pedy, temperatures are forecast to be a little milder on the coast, reaching 32C in Adelaide and 25C in Mount Gambier.

Fires are continuing to burn across the country. Picture: SA CFS

The heatwave is encouraging further flames in the state, Picture: Morphett Vale Country Fire Service
The stifling heat will continue as it jumps across the border to Victoria and NSW.

“Temperatures (will) vary quite a lot across the state,” Bureau of Meteorology senior meteorologist Angus Hines said.

“We’ll see low 20s in the southern half but high 30s through some areas of the north.”

Those in Mildura in Victoria’s northwest and Albury-Wodonga on the NSW-Victoria border are set to cop a maximum of 37C, while temperatures will reach the mid-30s in Victoria’s central region.

In Melbourne, temperatures are forecast to be much chiller than the rest of the state, with cloudy skies and a top of 23C.

It’s perfect beach weather for residents in parts of NSW. Picture: NewsWire / Andrew Henshaw
It’s a different story in NSW, with warm and sunny conditions expected in the majority of the state.

“We’ll see most of our urban centres in the 30s and many places in the mid or even upper 30s,” Mr Hines said.

The suffocating heat will stretch from the Victoria border all the way to the west, with temperatures forecast to reach 38C in Bourke, Broken Hill and Dubbo.

Sydney will also feel the heat as the mercury tips to 33C on Thursday.

The heat is lingering for a few more days. Picture: Windy
Residents keen to hit the beach on Thursday may be in for a shock, though, as thunderstorms risk sweeping parts of the state in the afternoon.

“The vast majority of the day will be sunny, but as we get into the afternoon, a little bit of cloud will build and that will bring a low risk of a shower or thunderstorm to areas south of Sydney and also across our western districts,” Mr Hines said.

“But you’d be pretty unlucky if you happened to get some rain.