Authorities in South Australia have issued a public health warning ahead of an extreme heatwave forecast for southern and eastern parts of the country during the week.

Key points: The BOM is expecting temperatures to reach the high 40s towards the end of the week

The BOM is expecting temperatures to reach the high 40s towards the end of the week Many locations will endure several consecutive days of very hot weather

Many locations will endure several consecutive days of very hot weather Authorities have advised local residents to take the necessary steps to stay safe

Temperatures in the mid to high 40s are forecast for parts of South Australia, Victoria and New South Wales, increasing the bushfire risk.

Adelaide is expected to reach or exceed 40 degrees Celsius for four consecutive days, while Broken Hill and Mildura — which is predicted to hit 46C on Friday — are among several locations expecting five such days in a row.

Bureau of Meteorology (BOM) maps show large parts of the country under either severe or extreme heatwave conditions, intensifying mid-week.

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Senior forecaster Tom Boeck said while South Australia had so far been spared the extremely hot start to summer endured in Western Australia and the Northern Territory, temperatures in the high 40s would hit parts of the state including western districts and Eyre Peninsula.

"Australia-wide, certainly temperatures are right up there," he said.

"We're expecting heatwave conditions to be increasing through the week.

"With that, there will obviously be increasing fire danger … probably peaking towards the later part of the week with winds also increasing and that'll be ahead of a change."

Adelaide's run of sweltering weather will begin on Tuesday and will peak at 44C on Friday before that change brings milder conditions.

However, temperatures in remote parts of South Australia, such as the Nullarbor Plain, are likely to be significantly hotter and are forecast to approach 50C.

SA's State Emergency Service (SES) has issued a community readiness alert, advising residents of steps they can take to keep cool and stay safe, and SA Health is also urging people to prepare.

"Alcohol is not a hydration source. Essentially, water is the best thing … be mindful of that, especially as the Christmas party season starts to ramp up," said acting chief health officer Chris Lease.

"Heat stroke and heat exhaustion are a real risk. The body struggles to cope with temperatures above 35C.

"People need to monitor their hydration levels."

Northerly winds to fan extreme heat

Dr Lease said pregnant women, children, the elderly and people with chronic health conditions were potentially at the greatest risk.

He said heatwaves were associated with spikes in the number of ambulance call-outs and people attending hospital emergency departments.

Large swathes of the south-east have been urged to prepare for extreme heatwave conditions. ( Bureau of Meteorology )

Western Australia has recently undergone a blast of summer heat, but Mr Boeck said South Australia had generally been kept cooler by onshore winds.

"We have had relatively mild conditions for the southern parts of South Australia but the weather pattern looks like it's changing," he said.

"A high-pressure system over the Bight area has produced some onshore winds and kept temperatures mostly on the milder side.

"The high's going to be moving out into the Tasman Sea area so that'll see northerly winds coming in through South Australia."

He said he expected several towns to set new December temperature records.

"The hottest places will be through the Eyre Peninsula area and north-western parts of the state, so places like Ceduna, Tarcoola, Kyancutta — I'm expecting the temperatures through those areas will probably get to the 48C, 49C mark later this week," Mr Boeck said.

The SES said it was possible a code red would be declared to provide extra support for rough sleepers.

While many residents will be struggling to stay cool, some are hoping for a silver lining.

Adelaide Hills cherry farmer Wayne Altmann said the high temperatures could get the season back on track ahead of Christmas, after a cooler start.

"We are positioned quite well for heat, there's lots of leaf on the tree to guard against sunburn and most of our varieties are still yet to fully ripen, so the heat will probably help us ripen it up," he said.

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