IT'S already 37 degrees in some parts of Victoria but those expecting to cool down in air-conditioned comfort may be forced to sweat it out instead.

A third electricity generator in the state was shut down at Yallourn Power Station overnight as people suffered through sweltering temperatures.

About 5800 properties lost power on Thursday evening as the mercury struggled to dip below 30C. A spokesman for the State Control Centre said the cause of the outages was under investigation.

Victorian Energy Minister Lily D'Ambrosio told 3AW this morning that three units at the coal-fire power stations Yallourn and Loy Yang A were not working.

"This means we can't rule out brownouts," Ms D'Ambrosio said. "We have ageing coal-fired power stations. They are becoming less reliable."

The Bureau of Meteorology has revised its Melbourne forecast to 44C for Friday - the hottest maximum temperature since it hit 46.4C on Black Saturday in 2009.

The temperature in Mildura in the state's northwest was already at 37C at 8.50am today, although a cool change is expected from about 3pm.

The power outages will likely feed into the energy debate, after previous problems in South Australia were blamed on the state's high proportion of renewables.

Prime Minister Scott Morrison has said he wants to focus on "fair dinkum power", defined as reliable power that functions when the sun isn't shining and the wind isn't blowing.

Yallourn power station running at 50% capacity due to an unknown fault in one of the 3 working generators. We are screwed today. — Alice Noble (Tanner) (@Ally_Tanner) January 24, 2019

Yet another coal unit has failed at Yallourn Power Station... total 4 coal burners now inoperable across the #nem on one of Australia’s hottest days of the year #fairdinkumpower pic.twitter.com/2ErtntEwaA — Daniel Kirkwood (@danielkirkwood) January 24, 2019

EVERY AIRCON in Melbourne will be running today.



EVERY CHANCE power suppliers will have to “load shed” by cutting power to areas temporarily to balance supply & demand of power — Hot Cross Matt (@DoctorIMatt) January 24, 2019

Now that the sun is shining and it's the conventional power stations that are struggling, the debate is sure to hot up again.

There are mounting concerns about the ability of coal stations to cope with heatwave conditions.

Those on social media were already commenting "we are screwed today" as news of the power station failures spread.

Yesterday, Ms D'Ambrosio had urged households to ration their non-essential electricity use.

While "absolutely confident" there was enough energy supply to get through the night, she suggested turning off dishwashers, kitchen appliances, washing machines and pool pumps.

"What we are asking Victorians to do is just be aware and be mindful that the activities we undertake when we're home do put that further pressure on our energy system," she said.

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

A cool change is expected about 3pm, with a temperature drop of 15C in just 20 minutes forecast for Melbourne. The sharp change will bring gusts of up to 80km/h, thunderstorms and increased fire risk.

The extreme weather has also impacted other parts of Australia.

On Thursday, Adelaide CBD recorded the highest capital city temperature in the world when it reached 46.6C at 3.36pm, also breaking an 80-year record for the city's highest temperature of 46.1C set in January 1939.

But the mercury rose even higher across the state, with Port Augusta the hottest at 49.5C and the fourth hottest temperature ever recorded in South Australia.

The town was one of more than 20 locations to experience record-breaking highs, others including Ceduna at 48.6C, Port Pirie and Whaylla at 48.5C.

South Australians will get some relief in the form of a cool change on Friday, when a maximum of 31C is forecast for Adelaide.

- With AAP

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