Temperatures across southeast Australia are set to soar for at least the next four days, with some areas reaching 45 degrees.
Heat building over western NSW will push towards the coast over the weekend and early next week, forcing temperatures up to 16 degrees above average.
"We have a large high pressure system sitting out in the Tasman and inland trough and together they're acting to funnel in a hot northerly air mass into the state," the Bureau of Meteorology's Alexander Majchrowski said.
"We'll be experiencing severe to extreme heatwave conditions for much of the state over the next several days."
As a result, fire danger will be elevated on Sunday and Monday, with the RFS warning people should use the next few days to prepare.
"People need to take this seriously ... this season we haven't had these heightened fire dangers but we have now got to be prepared," RFS NSW director Peter McKechnie told reporters on Thursday.
Bega, on the south coast, is expected to reach at least 39 degrees on Monday, and western Sydney will be in the same range on Sunday and Monday.
Other parts of southern NSW could experience temperatures up to 45 degrees on Sunday and Monday, according to BOM.
Australia Day will be hot too, with temperatures unlikely to cool of until Wednesday.
Victoria will also see temperatures building to a peak on Monday, with most major centres in the state's north surpassing 40 degrees. NSW-Victoria border towns could endure temperatures up to 44 degrees.
A cool change is due to hit the state's southwest on Monday but won't reach the northeast before early Tuesday morning.
Victorians could find themselves sweating through the night on Sunday and Monday as temperatures stay high even after the sun sets.
BOM meteorologist Jonathan How said conditions over the weekend in southeast Australia would be the warmest since January 2020.
He said relief would not arrive for at least four days.
"Summer is really back in full across southeast Australia ... 15 million Australians will experience the scorching heatwave conditions over the next few days," Mr How told the Nine Network on Friday.
The heatwave has lifesavers on high alert, with Surf Life Saving NSW CEO Steven Pearce saying this is "probably the weekend we've been looking out for".
The heat is likely to drive people to the beach and a large swell will pose some "real challenges" for lifesavers and life guards, Mr Pearce told reporters. More than 1000 people have required rescuing since Christmas Day.
A severe to extreme heatwave is already besetting southwest Western Australia and western South Australia. A low intensity heatwave is expected to persist in SA until next week.
Australian Associated Press