Aussie towns among world's hottest in 'unheard of' heat

A string of Australian towns have become some of the hottest places on earth as a brutal heatwave pushes inland temperatures towards 50C.

Victoria's temperature record was broken on Tuesday as the towns of Ouyen, one hour south of Mildura, and Hopetoun both peaked at 48.9C.

Fierce heat has been gripping the nation's southeast for days, pushing temperatures at Renmark, in South Australia's Riverland region, to 49.6C.

Senior meteorologist Dean Narramore said many towns across the nation experienced record-breaking heat, marking their hottest day ever.

"There's some pretty longstanding records that have fallen," he told AAP.

The Victorian regional centre of Mildura hit a top of 48.6C, marking its hottest day on record.

It is a dubious honour, with Mildura mayor Ali Cupper describing the forecast conditions as "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 town of about 35,000 people is due to experience a series of 40C-plus days through to Saturday.

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

The hotel has become a natural refuge as local businesses closed early and residents sought relief at the lake or the pub.

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

The nearby town of Walpeup was first off the block, becoming the hottest town in the state at 12.30pm.

Temperatures peaked at 48.9C, also marking its hottest day on record.

While conditions are most extreme in the regions, the slow-moving heatwave did not spare cities.

After sweltering through one of its hottest nights on record, Adelaide got off to a scorching start on Tuesday, crossing 41C by 9.30am.

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

A cool change brought some reprieve to the capital shortly after midday and is set to deliver cooler conditions throughout the evening.

Melbourne got off to a slower start but temperatures continued to build, edging past 40C by mid-afternoon, before reaching a maximum of 42.7 in the early evening.

The city was on track to break its second-hottest January day on record, behind the 46.4C day before the devastating Black Saturday bushfires of 2009.

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

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

The weather bureau has warned gusty southwesterly winds will reach Melbourne on Tuesday night, with a risk of dry lightning from thunderstorms.

Paramedics have also issued a warning after responding to 11 cases of children being locked in cars over the weekend as temperatures soared into the 40s in Victoria.

While Victoria and SA have been the worst affected, elevated temperatures are being felt nationwide.

Extreme heatwave warnings are in place across large parts of NSW, including the Snowy Mountains, southwest slopes and upper western regions.

The outback town of Broken Hill broke its all-time record, hitting a high of 47.8C.

About 110km away, the tiny township of Fowlers Gap hit 49.1C.

Severe heatwave warnings are also in place for parts of the Northern Territory, the ACT and Queensland.