All-time records tumble as heatwave highs touch 50C

Scorching temperatures are roasting residents in much of Australia's southeast as records tumble in the drawn-out heatwave.

The mercury is expected to soar in much of Victoria and South Australia on Tuesday, with the Bureau of Meteorology warning the extreme heat could reach all-time highs.

Melbourne will swelter through its hottest day since the devastating Black Saturday bushfires in 2009 with a maximum temperature of 44C expected.

Mildura, in the state's far north, is forecast to reach 49C, while Broken Hill in NSW is expected to hit 47C, with both locations set to break temperature records.

"If Mildura does reach (that high), it will be the hottest temperature recorded in Victoria," senior meteorologist Dean Narramore told AAP.

"Pretty much all observation points near the SA and Victorian border are either approaching or likely to exceed January records and a few locations could also break their all-time records."

Ouyen, a rural township of about 1100 people in Victoria's north, is also set to become one of the hottest towns on earth, with a maximum of 49C forecast for the afternoon.

While residents were familiar with extreme heat, Mildura mayor Ali Cupper said the forecast was "unheard of".

"If you're out in the heat, a couple of degrees can be the difference between life and death," she told AAP.

The region is set to swelter through multiple days of 40C-plus heat until conditions moderate from Sunday.

Ms Cupper urged locals to put out water for local wildlife, stay indoors when possible and monitor for the latest weather updates.

Emily Dalton, publican at Ouyen's Victoria Hotel, said the heat has been building steadily each day, with conditions becoming especially punishing by late afternoon.

"By four or five o'clock it's tough to be outside," she said.

The hotel has become a natural refuge as local businesses closed early while residents headed to the lake or the pub.

"Bring your dogs, bring your cats - the beer is cold and the food is good," Ms Dalton added.

Statewide fire bans are in place for Victoria and SA as multiple blazes burn out of control in the tinderbox conditions.

The temperature in Adelaide reached 44.7C on Monday, topping the city's previous Australia Day heat record from 2006.

Ceduna, northwest of the state's capital, climbed to 49.5C, surpassing its previous record temperature, while NSW town Dubbo reached 46.1C to record its hottest January day.

In Victoria, a blaze in the Otways continues to burn out of control after jumping containment lines on Saturday, threatening the small community of Gellibrand.

There have been unconfirmed property losses within the fire grounds.

The weather bureau warns gusty southwesterly winds will hit Melbourne on Tuesday afternoon, with a risk of thunderstorms producing dry lightning.

A cooler change will begin to move through Victoria and South Australia on Wednesday.

However, inland parts of the southeast won't experience much of a reprieve, with temperatures in Victoria's north expected to reach 40C for seven days.

Deputy incident controller Alistair Drayton said the gusty change was worrying, with authorities urging residents living in Gellibrand and surrounding areas southwest of Melbourne to evacuate.

"The conditions mean the fire could spread quickly and unpredictably," he said.

"We strongly recommend people in affected areas to enact their bushfire plan, and leave early to protect themselves and their families."

The heatwave conditions have also sparked a warning from paramedics, who responded to 11 cases of children locked in cars over the weekend as temperatures soared into the 40s in Victoria.