Swathes of NSW, Victoria and South Australia have endured periods of 12°C above the March average during a sweltering weekend.
Melbourne raced to 35°C and a touch more in its western suburbs on Saturday, while Adelaide peaked just short of 40°C.
It's been hotter still inland, with Mildura in Victoria's northwest hitting 41C and South Australia's Renmark and Port Augusta 42°C on Saturday. The hottest place in the nation was at Ceduna, which reached 44.1°C.
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A change has brought cool comfort to South Australians and Victoria's northwest, but Sydney is still bracing for a Sunday maximum of 38°C after enduring an uncomfortable overnight minimum of 26°C.
"The heat will really be across NSW on Sunday," Bureau of Meteorology senior forecaster Jonathan How said.
"Most areas will see temperatures of six-to12 degrees above average and that means temperatures will soar into the mid-to-high 30s inland, even pushing 40 degrees through western parts of the state."
Along the coast, residents can expect high 30s including for the Illawarra and Hunter Valley. In Sydney, gusty winds will accompany the swelter.
"The change is currently tracking across southwestern parts of Victoria and is due in Melbourne and Hobart later this morning and then we will see it push through the rest of Victoria," Mr Howe said early on Sunday.
"For NSW, we will see the cool change push up the south coast and it's due in Sydney late tonight, around midnight, with a southerly buster to bring some gusty winds as well."
Showers, cold winds and thunderstorms are likely to follow.
High and possibly damaging winds are likely in alpine areas of Victoria and NSW into Monday, as is snow across Tasmania's highlands.
A grassfire propelled by heatwave conditions meanwhile destroyed one home and damaged another at Montrose on Melbourne's outskirts on Saturday night.
South Australian firefighters are also battling blazes near Katarapko Island, northeast of Adelaide, and in the state's lower southeast near Lucindale.
