Temperatures will begin to climb in the three states from Christmas Day, according to Weatherzone.
Adelaide is forecast to reach 25 degrees on Christmas Eve, then 32 degrees on Christmas Day and 38 degrees on Boxing Day.
By December 27, the temperature could tip 40 degrees.
In Melbourne, maximum temperatures could jump from the mid-20s on December 24 to 30 degrees by Boxing Day.
The city could potentially reach the mid-30s around December 29 and 30.
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Meanwhile, Hobart could see temperatures reaching the high 20s around December 27 and 28, which could climb six to eight degrees above average for this time of year.
The hot spell is set to hit after weeks of unusually cold temperatures in south-eastern Australia.
Melbourne has endured its coldest first half of December since 2001 and Victoria registered its lowest summer temperature on record on December 14, with the temperature dipping to -5.4 degrees at Mt Hotham at 5:09am.
Sydney skies darkened, a warning of the sudden storm to come