A power blackout has left two towns in central Victoria to sweat out a day of extreme heat without air-conditioning, refrigeration or even a public swimming pool.

Key points: A faulty underground power line is believed to have caused a blackout affecting thousands of homes in central Victoria

A faulty underground power line is believed to have caused a blackout affecting thousands of homes in central Victoria Oppressive conditions are also expected overnight

Oppressive conditions are also expected overnight Extreme and severe fire danger ratings are in place for much of the state for tomorrow

About 2,700 houses in Nagambie and Avenel were affected by the blackout, which came on a day when extreme heat warnings had been issued for most of the state.

Much of the state was baking, with maximum temperatures in the low 40s, while Adelaide hit 46.6C, beating Melbourne's decade-old temperature record for an Australian capital city.

The mercury climbed above 40C in Melbourne at 5:30pm, while in Mildura, in the state's north-west, it reached 44 degrees

AusNet said it was not the heat, but a fault with an underground power line that was to blame for the blackout affecting Nagambie and Avenel.

The power outage caused the closure of public pools in the towns and the local council set up a relief centre to help people escape the heat.

It also said it was working with Hume regional emergency response teams to organise a place for residents to escape the heat as well as setting up drinking stations around the affected towns.

Despite the extreme heat, the Nagambie pool could not open due to the blackout. ( ABC News: Mahalia Dobson )

Medicines affected

For Leah Wilson and her diabetic daughter, the blackout was far more than an inconvenience.

Ms Wilson had to take her daughter to the emergency department of the hospital in Seymour.

"Her insulin clouded over because it couldn't be kept cool," Ms Wilson said.

Other items in her fridge were also affected, and she hoped AusNet would provide customers with some form of compensation.

"We're going to have to throw out meat and veggies and fruit," she said.

"I contacted admin at AusNet to find out if we could possibly get any sort of compensation and were promptly told that only if we had shopping receipts.

"Who goes shopping and thinks, 'Oh, I must keep that, just in case the power goes out'?"

One resident with a generator took to the local community Facebook page to offer up her fridge's spare shelf space to help others prevent their food from spoiling.

'It's going to be oppressive'

There will be little relief from the heat throughout the state overnight, with lows of 31C forecast in places like Mildura, and 29C tipped for Melbourne.

Temperatures will then ramp up again tomorrow, with Melbourne to reach around 44C before a cool change hits mid-afternoon.

"Heat is our number one natural hazard killer in Australia," said the Bureau of Meteorology's state manager, Dr Andrew Tupper.

"Understand that heat is a killer … that needs to make a difference in the way we behave."

He said it has been "a particularly hot summer", with December the hottest on record nationally — and plenty more heat to come.

"January is lining up to be the same … Summer overall is going to be one of our hottest summers," he said.

Australian Open heat rule in place

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Today's heat has affected the Australian Open tennis tournament in Melbourne.

Organisers said the heat rule was put in place for wheelchair and junior events taking place on the exposed, outdoor courts earlier today.

The roof of centre court, the Rod Laver Arena, was also closed for the afternoon's women's semi-final between Petra Kvitova and Danielle Collins.

Because of the heat, players were granted extended breaks between the second and third sets.

Fires controlled today, extreme threats tomorrow

Firefighters work on the Huon Hill fire, in north-east Victoria, yesterday. ( Facebook: Wodonga West Fire Brigade )

Despite today's extreme conditions, firefighters in the north-east of the state have been able to contain a number of fires caused by a spell of dry lightning on Tuesday.

Their major concern now is what tomorrow will bring, with strong winds and highs of 45C being forecast.

"Friday's a horrible day with a late change, with wind, with a landscape that's been baking for a number of weeks now," CFA operations manager Paul King said.

"We expect that we're going to have really difficult fire danger."

A total fire ban is in place across the entire state tomorrow, with the fire danger rating at extreme for the Northern Courty and Mallee districts.

The rating is severe for the Wimmera, South West, Central and West & South Gippsland regions, and very high in the remaining areas of Gippsland.

Firefighters urged residents to be mindful of the potentially dangerous fire conditions and avoid activities like angle grinding and slashing.

Authorities were also concerned that careless visitors over the Australia Day long weekend could put extra pressure on fire crews, with concerns around campfires and fireworks.

"We've really hit a critical point in the bushfire season," said Aaron Kennedy, the Department of Environment, Land, Water and Planning's assistant chief fire officer for the Hume Region.

"So we're really asking people to vigilant, particularly if they're in our forests or on the river this Australia Day weekend.

"Any fire that starts, we're seeing it's going to be difficult to control."