Temperatures in the high-40s are expected in parts of Victoria and NSW, while the Northern Territory faces severe to extreme heatwave conditions for much of the next three days.
While Adelaide sweated through a 40C day on Sunday, Victoria and NSW were set to endure a night of minimum temperatures between 12 and 16 degrees above the average.
"A hot day leading into a very warm night leading into a very hot day," Bureau of Meteorology senior meteorologist Lincoln Trainor told AAP on Sunday.
Spots for cooling off will be in high demand as temperatures soar around the country. (Dan Himbrechts/AAP PHOTOS)
Hot, dry and gusty north-westerly winds are expected to bring high bushfire risks to parts of NSW and extreme bushfire risks to most of Victoria.
The heatwave marks an early start to summer and could break a number of December records in parts of the nation.
Extreme fire danger has been declared in Victoria's central and western districts while Melbourne, with an expected forecast of 41C, will face its hottest day since January 2023 and its hottest December day since 2019.
The mercury is expected to hit 46C in Mildura in the state's northwest, 45C in Swan Hill and 44C in Horsham.
"It's a hot, dry and windy day," Mr Trainor said.
"The main risk ... is definitely going to be the elevated fire danger."
Total fire bans have been declared across most of Victoria with incident management teams and firefighting aircraft on standby in critical regional areas.
Extreme fire danger has been declared in Victoria's central and western districts. (Con Chronis/AAP PHOTOS)
While Sydney can expect a mild start to the week with a 28C maximum forecast for Monday, parts of inland NSW could face even more extreme heat than Victoria.
A maximum temperature of 47C is expected in Wilcannia, in central northwestern NSW, and 46.5C in Ivanhoe, about 180 kilometres further south.
A cold front is expected to reach western Victoria around 3pm on Monday, bringing relief to Melbourne and central parts of Victoria by around 9pm.
"There will be still a risk of storms in the east during Tuesday and showers in that south-westerly," Mr Trainor said.
"But essentially, all that heat goes away on Tuesday."
23°C