'Very serious' heat warning with records under threat

Record-breaking heat is set to scorch Australia's southeast, prompting urgent heatwave warnings for Australia Day.

Victorians are sweating through what could be the state's most severe heatwave in nearly two decades, while South Australia's capital readies for potentially its hottest Australia Day on record.

Melbourne will enjoy some reprieve on Monday, with a forecast top of 30C, before temperatures surge back into the low 40s from Tuesday.

Towns across Victoria's north face the prospect of seven consecutive days above 40C, with tops of 47C in Swan Hill and Mildura forecast for Monday.

The state could record its hottest day ever on Tuesday, as temperatures edge towards 50C.

The northern town of Ouyen is forecast to hit 49C, with authorities likening conditions to those seen during the Black Summer of 2009.

"This is a very serious set of weather conditions," Emergency Management Commissioner Tim Wiebusch said on Sunday.

"We haven't seen heatwave conditions like this in Victoria for almost 20 years."

Meanwhile, extreme heat will blanket much of South Australia, including in Adelaide, which is set to reach a maximum of 45C in the mid-afternoon.

If that peak is reached, it would mark the city's hottest Australia Day on record, Bureau of Meteorology senior meteorologist Jonathan How said.

The scorching temperatures are being driven by northerly winds, funnelling blistering inland heat towards coastal areas.

Temperatures climbed above 48C in Port Augusta and Tarcoola on Saturday, before easing slightly on Sunday.