The Bureau of Meteorology says the week's hot weather could break heat records nationally and in individual states.
National heat records could topple this week as parts of Australia swelter through temperatures approaching 50C.
Adelaide is expected to reach a high of 40C on Tuesday, with the maximum to stay in the low 40s until the mercury peaks at 44C on Friday.
That day will also bring a forecast top of 41C in Melbourne, while temperatures in Canberra will soar to 43C on Saturday.
"Australia's warmest day on record occurred in January 2013, when the average maximum temperature across the continent was 40.3C," climatologist Dr Blair Trewin said on Monday.
"We're closely monitoring the development and progression of this heat but, based on current forecasts, we could see that record broken this week."
Meteorologist Sarah Scully said severe-to-extreme heatwaves in several areas will increase the risk of heat-related illnesses.
"It's important people follow the advice of health authorities, stay up to date with the latest forecasts and warnings from the bureau and remember to check in on any vulnerable family and friends," she said.
In South Australia, the state government has issued a code red to boost homelessness services across Adelaide until Friday.
"Keeping vulnerable South Australians safe and well in the extreme heat forecast over the next few days is our priority," Human Services Minister Michelle Lensink said.
Baptist Care SA is among the organisations that will stay open around the clock to give homeless people a place to keep cool and well-hydrated.
Temperatures are expected to be even hotter in regional areas, with the town of Oodnadatta, in South Australia's Far North, among those predicted to endure five consecutive days of 45C or hotter, including three consecutive 47C days.
Other places that could experience temperatures in the high 40s are parts of the Eyre Peninsula, Riverland, far west and northwest of the state.
Senior SA forecaster Paul Lainio said state records could be under threat during the week of "exceptional heat".
"The record for South Australia in December is 49.1C in Moomba back in 1972," he told reporters in Adelaide.
"We need to get up to those sort of levels to see records. It is possible, though, in some of those locations in the west and north of South Australia."
Mr Lainio said Friday would be the "danger day", with a cool change expected to move through later on.
"That might bring us some joy but not before we have some hot, dry windy conditions... very dangerous fire weather conditions to finish things off," he said.