It was already forecast to be blisteringly hot across southern Australia over the next few days. Now it's getting worse.

Conditions are set to deteriorate with the potential for a November temperature record that's stood for 125 years to be broken in Melbourne tomorrow.

The Victorian capital had been forecast to reach 34C today and Thursday but a reassessment by forecasters has suggested it could now approach 40C tomorrow, reports News.com.au.

That would see Melbourne only a little cooler than today's scorching high in Adelaide of 42C.

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Overnight on Thursday in Victoria could be "oppressive," said the Bureau of Meteorology.

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Total fire bans are in place across much of South Australia with catastrophic fire conditions, the most severe bushfire rating, in place around Adelaide.

Adelaide is set to reach 42C today ahead of the front. Expect dust haze and winds of up to 45km/h which could exacerbate any bush fires. But on Thursday the maximum could drop by as much as 20 degrees to a mere 25C.

'UNUSUAL' CONDITIONS IN MELBOURNE

Temperatures in Melbourne are set to soar. They often ramp up before a cool change comes through but a change in behaviour of the coming front has led to a big jump in where that maximum could land.

"The cool change that is tracking from Western Australia and across the south looks to be moving a bit slower than previously forecast and that would allow Melbourne to be very hot ahead of an afternoon or early evening cool change," Sky News Weather meteorologist Rob Sharpe told news.com.au.

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"Melbourne may eclipse its November temperature record of 40.9C on Thursday, set in 1894."

Look out for 34C today and then as high as 39C in the city and 40C plus in the suburbs tomorrow. Many inland towns will reach 40C.

Bureau of Meteorology Victorian senior meteorologist Richard Carlyon said northerly winds will push desert air southward.

"Normally this time of the year we start to see summer weather above 30C but it is unusual to see temperatures in the high thirties," he said.

Overnight between Wednesday and Thursday, the mercury might not fall below 25C. To put that into context, the average November maximum temperatures in Melbourne is three degrees lower at 22C.

"It will an oppressive night," Mr Carlyon said. At 3am it will feel more like 3pm.

On Friday, once the cold front blows through, the high will top out at just 20C – a massive decrease.

AROUND THE OTHER CAPITALS

A forecast high of 23C in Hobart today. Then 30C on a sunny Thursday when Tasmania could also see some showers. A drop to the low twenties from Friday.

The nation's capital is getting hotter and hotter. Canberra might not see a day below 30C until Sunday with 32C today and then 39C on Thursday. Ratings of very high for fire danger are in place for the ACT this week.

Sydney should be much cooler today following a scorching Tuesday with terrible air quality which saw the mercury rise to close to 40C in the city's west and 33C nearer the centre.

A cloudy Sydney is looking at just 25C with minimums in the high teens, setting a pattern for the next few days. There may be some rain on the way, but not until the weekend.

Warm in Queensland with a high of 33C today in Brisbane, and then around 29C for the rest of the week. Very high fire danger in the south east of the state

Perth is looking at 21C today with some patches of rain and the creeping back into the thirties by the weekend. A stormy week in Darwin with highs of 35C and overnights dropping to 26C.