Much of Australia's southeast is set to endure at least two warm nights in a row as a heatwave sets in.
Severe conditions have hit South Australia - including Adelaide - and regional Victoria on Sunday, with some regions copping temperatures above 40C.
The rest of Victoria, Tasmania and western and southern parts of NSW will likely see the hottest temperatures on January 25, while the mercury will climb the highest on Tuesday for eastern NSW and Greater Sydney.
In NSW on Sunday afternoon, temperatures had already risen to 37C in Sydney's west and 41C at Hay in the state's western Riverina region.
Bureau of Meteorology senior meteorologist Gabrielle Woodhouse said heat records were unlikely to tumble over the coming days and people across SA, Victoria and NSW were unlikely to sleep easy.
"The important part with the heat we're seeing is those overnight temperatures remaining particularly warm," Woodhouse told reporters on Sunday.
"Over parts of the southern inland in particular, we're looking at (overnight) temperatures anywhere in the high 20s, even touching 30C ... so very warm."
Woodhouse added that the heat - including overnight temperatures - was unlikely to subside in NSW until Tuesday night or Wednesday morning.
On Australia Day on Tuesday, temperatures would fall to between 38C and 41C in western Sydney and around 32C in the city due to a sea breeze. Canberra is also forecast to swelter, reaching the mid-30s before an evening shower.
"What we're seeing with this heatwave is multiple days of those really high maximum and minimum temperatures which can be quite difficult to manage when we haven't had a history of that this summer," Woodhouse said.
While winds are not expected to be as high as previous days of extreme fire danger, several districts in southeast Australia remain on high alert for blazes.
A fire weather warning is in place on Sunday for South Australia's mid-north, Mount Lofty Ranges, Yorke Peninsula and lower southeast districts.
A marine wind warning is also in place for large stretches of the NSW coast on Sunday and Monday, with Woodhouse warning of northerly gusts.
In Tasmania, a very hot day is forecast for Monday, with temperatures hitting the mid-30s across much of the island's east and south. However a cool change is expected to reach Hobart around midday.
NSW Ambulance's Kay Armstrong told reporters on Sunday that Australians needed to stay hydrated and look out for each other during the heatwave.
"You don't want to be out in the middle of the day," Armstrong said.
"Make sure you minimise exposure ... stay in the cool."
The heatwave also has lifesavers on high alert, with Surf Life Saving NSW chief executive Steven Pearce saying this is "probably the weekend we've been looking out for".