Adelaide is shaping up as the hottest capital city for Christmas Day, with the mercury tipped to soar towards 40 degrees Celsius, with an extreme fire danger forecast.
The Bureau of Meteorology predicts a maximum of 37 degrees for the South Australian capital on Friday, before a cool change sweeps through in the early evening.
With the hot conditions accompanied by dry northerly winds, it is likely to be the state's the most dangerous Christmas Day for bushfires in decades.
Adelaide residents are currently sweltering through a stifling Christmas Eve that's expected to hit 38 degrees.
SA Country Fire Service (CFS) state coordinator Leigh Miller said he couldn't remember a Christmas Day in Adelaide with a worse fire risk.
"It's going to be a hot day, a windy day and even if there's no fire bans, it's still going to be a significant fire day with those types of conditions," Leigh told the ABC.
"What we're likely to see is that the most dangerous fire conditions are after the change comes through, even as it gets cooler, but winds come up, we see the most dangerous fire conditions."
He said the CFS and emergency crews would be on high alert with some additional personnel rostered on in case of a bushfire outbreak.
BoM's acting regional director, John Nairn, has forecast temperatures in SA to reach the high 30s or low 40s with northerly winds tomorrow.
That has prompted a CFS warning that total fire bans could be declared for large swathes of the state, meaning that many South Australians could have to forego a Christmas barbecue.
The CFS said it would confirm the number of regions with total fire bans in place for Christmas Day by about 4.30pm today.
"Unfortunately that means that some people may not be able to have their Webber kettle barbecues and those types of things but I'm sure there are other arrangements that can be made," CFS' Rob Sanford said.
Meanwhile, Melbourne residents are also in for a hot Christmas Day, with the temperature tipped to hit 33 degrees under sunny skies.
Hot and dangerous bushfire conditions are tipped for some parts of Victoria, especially in the Indigo Valley about 300 kilometres from Melbourne where firefighters are working to control blaze that remains at watch and act alert level.
Victoria's Country Fire Authority says firefighters are "making good progress" to contain the blaze, but that it remains out of control.
The Barnawartha fire has already burned over 9,000 hectares and claimed a number of houses.
Across the border in NSW, cooler conditions are predicted in Sydney for Christmas Day, with a maximum of 25 degrees forecast.
Weatherzone meteorologist Tristan Meyers said there was about a 50 percent chance it would rain in Sydney.
"This is all due to the position of a trough that is moving across NSW," he told Fairfax Media.
"If the trough would speed up we could have showers for most of the day or if it slows down then we could have only scattered, light showers.
"Right now it's looking like we could have some showers."
In Brisbane, a shower or two is forecast, with a maximum temperature of 27 degrees tipped for the Queensland capital.
Perth is in for a sunny Christmas Day and a maximum of 27 degrees, while Tasmania is expecting hot and sunny conditions and a max of 33 degrees.
Canberra and Darwin, meanwhile, are both forecast to reach a maximum of 29 degrees.