Victoria's north-west is on track to beat the record for the most consecutive days over 40 degrees Celsius in December, with a heatwave set to bring temperatures not seen in almost a century.

Key points: Health departments in NSW, SA and Victoria issue warnings of extreme heat

Health departments in NSW, SA and Victoria issue warnings of extreme heat The Victorian town of Kerang set to face the hottest run of weather since 1920

The Victorian town of Kerang set to face the hottest run of weather since 1920 Heatwave is caused by a high pressure system off the east coast trapping in the heat

The Bureau of Meteorology (BOM) has forecast temperatures will exceed 40 until Sunday in Kerang, with little relief in sight until early next week.

A high pressure system off the east coast has trapped in the heat, pushing up temperatures across the southern parts of the country.

In Victoria, Kerang reached 40C on Boxing Day while Mildura and Swan Hill recorded temperatures above 40C.

Port Augusta in South Australia reached 44.3C on Boxing Day while temperatures in towns in the central west region of New South Wales soared into the high 30s.

Health departments in New South Wales, Victoria and South Australia have issued warnings for extreme heat, urging people with medical conditions, young children and the elderly to take particular care.

Kevin Parkyn, a senior forecaster for BOM, said the weather in the Mallee region looked set to break records going back to the end of World War I.

"We're looking at five consecutive days above 40 degrees in [Kerang] and we have to go back to about 1920 in the record books when we saw four consecutive days in December," Mr Parkyn said.

"It's hot in the north, heatwave conditions."

The BOM has forecast a severe heatwave across the southern parts of Australia.

Beren Bradshaw, a senior meteorologist for BOM, said there might be a slight reprieve next week but the hot temperatures looked to continue into January.

"There is a weak change that's forecast to come through with one or two millimetre of rainfall also forecast for early next week," she said.

"But beyond that there is still a little bit of uncertainty.

"We are getting right into the middle of summer so there is a good chance the hot conditions will continue."

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Mr Parkyn said temperatures would be the highest seen since March this year in Melbourne, where Australia is playing India in the Boxing Day Test.

"We're looking at … a revised maximum temperature for Melbourne to be 36C," he said.

"It will be pretty hot at the MCG and those conditions will continue on Friday, when our temperature of 34C is expected."

Jeremy McAnulty from NSW Health said it was important vulnerable people prepare for the severe weather.

"The most prone are those with underlying medical conditions and the elderly and small babies, so if you are one of those people or if you have neighbours or relatives who fit into those categories, check on them regularly to ensure they're keeping their fluids up and not doing unnecessary chores when it's really hot," Dr McAnulty said.

Canberra has not been spared from the heatwave, with temperatures forecast to exceed 34 degrees every day into the new year.