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 40-day on Sunday, Victoria and NSW were set to endure a night of minimum temperatures between 12 and 16 degrees above the average.

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.

Total fire bans have been declared across most of Victoria with incident management teams and firefighting aircraft on standby in critical regional areas.