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MELBOURNE - Emergency authorities in Australia's south-east are on high alert as a heatwave sweeps through the region.

With the mercury tipped to hit 44degC in Adelaide on Wednesday, authorities urged South Australians to take extreme care.

The SA chief medical officer warned elderly people, babies and young children were most at risk, with sustained temperatures expected to stay above 35 degrees in the state until next Monday.

Thoroughbred Racing Australia said an eight-race meeting set down for Gawler on Wednesday was abandoned because of extreme weather conditions.

Shortly after 11am CDT the temperature was already 43.4 degrees in Adelaide city, and 44.6 in Edinburgh, in the northern suburbs.

In Melbourne, the Bureau of Meteorology tipped a scorching 41 degrees on Wednesday, with temperatures about 44 degrees in the north-west part of the state.

By noon AEDT the temperature in the city had reached 38.7 degrees, and 40 degrees in Laverton, in Melbourne's west.

Victoria's Department of Sustainability and Environment was called to a number of fires, but all were controlled.

The Country Fire Authority was also called to several blazes, but all were safe or controlled.

Senior forecaster with the Bureau of Meteorology, Dean Stewart, said the hot weather was likely to continue for the remainder of the weak.

Although Saturday's expected top temperature of 40 degrees in Melbourne may be downgraded slightly, Mr Stewart said those expecting relief from the heat would have to wait until Monday.

On Monday the expected maximum forecast for Melbourne is 27 degrees.

Melissa Veal from the Country Fire Service in South Australia said so far the state had been fortunate during the current heatwave.

"We are on high alert today. We have our state co-ordination centre ready to go in case anything does happen," she said.

Today is a day of total fire ban across much of Victoria and all of South Australia.