Thousands of properties are without power in north central Victoria where temperatures are expected to exceed 40C.

More than 2700 AusNet customers have been hit with a power outage near Nagambie on Thursday morning.

The underground cable fault might not be fixed until 5pm.

READ MORE Australia set for more heatwaves and fires as climate change hits: report

The outage comes as residents in South Australia, Victoria and Queensland brace for another day of record-breaking heat.

Adelaide is forecast to hit 45C on Thursday, and the Bureau of Meteorology says the city's 80-year-old record high of 46.1C is within reach.

"You wouldn't rule it out," forecaster Jonathan Fischer said.

The Australian Energy Market Operator is trying to allay fears Victoria and South Australia might suffer a power shortage over the next few days.

Chief executive Audrey Zibelman said there were enough reserves to see through the heat.

"We first go to the generators that are in the market, then we go into our reserves and we carry enough reserves to lose two units and then, only then, we would ask for people to reduce their usage, but that's the last point and we're not even close to that yet," she told ABC Radio.

'Nowhere is going to escape'

The heat will intensify in the South Australia's regional centres, with a high of 49C expected at Port Augusta, and 47C at Port Pirie and Roxby Downs.

"Nowhere is going to escape," Mr Fischer said.

"We will see records broken tomorrow, with many locations forecast to be near their maximum all-time record."

Residents living in the outback town of Birdsville endured 10 consecutive days of 45C and above until Monday, exceeding the previous six-day run that occurred in the same town in 2004 and 2013.

AAP-EPA-MIGUEL A. LOPES

Bureau of Meteorology forecaster Mark Trenorden said the Australian record of 13 days occurred in Marree in South Australia in January 1973.

"This is the longest spell of 45 ever in Queensland," he told AAP.

State weather records have also fallen in Cloncurry and Camooweal, where the mercury has hit 40 degrees or above for the past 39 days, with no reprieve in sight.

'Oppressive' nights

Melbourne is forecast to reach 36C on Thursday, while low 40s are forecast for other parts of the state.

The city will swelter through its hottest night in a year with the temperature dropping to just 29C, before hitting a top of 43C on Friday.

AAP

"The overnight minimum temperatures are nothing short of oppressive unfortunately," the Bureau of Meteorology's Richard Russell said.

Authorities are asking people not to take risks during the hot weather, including leaving children in cars or swimming at unpatrolled beaches.

"Let's take something from these drowning deaths. Twenty people have lost their lives, that's 20 families and friends of those people that have been impacted," Emergency Management Commissioner Andrew Crisp said.

A cool change is expected late on Friday, but it will bring wind gusts of up to 80km/h, thunderstorms and an increased fire risk.

Fire crews are on stand-by while extra paramedics and ambulances will be on call to assist those struggling in the heat, Mr Crisp added.