BoM forecasts post-Christmas heatwave for southern Australia

While much of Australia is tipped to hover around the 30 degree mark for a welcome sunny Christmas, the days following it will see the heat turned up in south-eastern WA, western NSW and large parts of SA and Victoria.

Bureau of Meteorology senior meteorologist Jonathan How said the incoming weather "will come as a bit of a shock to people" in Australia's southern states.

"Especially given how cold it was at the start of this month," he said.

Mr How said the country could expect temperatures to "ramp up" from Friday, starting off in southern WA and moving into SA on Monday and into Victoria Tuesday and Wednesday.

Adelaide will be hit the hardest of the big cities, with 41 degrees being forecast for the SA capital on Tuesday December 27 - the first time above 40 for the city this summer.

Tuesday will also see parts of SA reach highs of 45 degrees for "a run of very hot days" for remote parts of the state east of Kalgoorlie.

The week will see a couple of days in the low 40s for inland areas pushing into western Victoria and south-western NSW.

Temperatures are expected to peak at 35 degrees for Melbourne on Tuesday

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Mr Howe said cooler conditions are expected across the south-east of the country for Thursday and Friday ahead of New Year's Eve.

"It is looking mild to warm for New Year's Eve and a little bit cooler near the east coast," he said.

"New Year's Day we could potentially see the start of another warm spell, depending on how much heat moves down from WA.

"Early stages but it is looking like another potentially hot start to 2023."

Elevated fire dangers are expected for the southern coast in SA and WA with both heat and gusty winds.