More than half of the Australian population is in the grips of a heatwave with temperatures expected to peak at 14C above average in most of the nation's southeast cities.

Adelaide is forecast to hit 41C on Sunday, while Melbourne is set to reach 35C on Sunday and 40C on Monday before a cool change arrives in the evening.

Canberra will also be hot, with temperatures of 38C expected on Sunday and Monday, and 37C on Tuesday, while Hobart is forecast to hit 34C on Monday.

Sydney will be slightly cooler than the rest of the southeast with a top of 33C expected on Sunday.

Weather bureau meteorologist Jonathan How said it was the first big heatwave of 2021 and it was due to a very hot air mass that had been sitting over Western Australia for a few weeks.

"More than half of the Australian population is currently in the grips of a heatwave," he told ABC News Breakfast on Sunday morning.

Sydneysiders cool off at Coogee Beach on Saturday. Picture: NCA NewsWire/Jenny Evans
Sydneysiders cool off at Coogee Beach on Saturday. Picture: NCA NewsWire/Jenny Evans

"People in Perth would be well aware of the very high temperatures and that is moving into the southeast, giving pretty much all southeast cities up to 14C above average temperatures for this time of year."

Mr How said Oodnadatta in northern South Australia was the hottest place on Saturday at 44.9C, while western Sydney reached the high 30s, Adelaide 39C and the rest of South Australia and NSW hit the low 40s.

And he said there was plenty more heat to come, with "stifling conditions" over the next few nights likely to see the temperature stay above 30C until well past midnight in Adelaide and Melbourne.

"Inland, we won't see the temperature get below 30C, 31C, and that makes it is difficult for the body to recover and that's why heatwaves are very dangerous," Mr How said.

A packed Melbourne beach on Saturday as temperatures reached 30C. Picture: NCA NewsWire/Ian Currie
A packed Melbourne beach on Saturday as temperatures reached 30C. Picture: NCA NewsWire/Ian Currie

"As you make plans for this (long) weekend, remember to factor in the heat and any fire weather warnings. Keep up to date with the forecast, stay hydrated and stay safe."

A cool change is expected to sweep across South Australia and western Victoria early on Monday afternoon, dropping the temperature to the mid-20s on Tuesday.

But Canberra and Sydney will have to wait until at least Tuesday evening for some relief from the heat.

Inland NSW will remain hot and the heat will rebuild across Western Australia with Perth expected to reach the high 30s again by next weekend.

Originally published as 'Scorcher': Half of country in heatwave


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