Parts of the ACT, NSW, South Australia and Victoria are set for 40C maximums on Sunday, the same scorching temperatures that many West Australians endured last week.
Among the capital cities, Sydney is likely to face the toughest test with temperatures in western suburbs such as Parramatta and Penrith expected to reach 39C.
Melburnians will feel a top of 38C, while people in Adelaide are readying themselves for 36C.
The UV index in each city will reach extreme levels, with sun protection recommended until about 5.30pm.
Health practitioners have encouraged people to keep cool and covered and avoid sun exposure, which can cause pain as well as lead to skin cancer.
Staff at the Women's and Children's Hospital in North Adelaide have treated 15 patients for severe sunburn in recent weeks.
Doctor Bernard Carney said parents and carers could not be too cautious about children being in the sun.
"It is incredibly distressing for children to be treated for sunburn. They are often in severe pain and require frequent dressing changes," he said.
"No parent wants to see their child suffer, especially from something that's preventable."
Dr Carney encouraged people to keep hydrated, wear a hat and loose clothes, and use sunscreen.
His comments echo the views of Australian of the Year recipients and melanoma researchers Georgina Long and Richard Scolyer who urged Australians to change their attitudes to sun exposure, particularly tanning.
Heatwaves could be dangerous for everyone's health, said NSW's health department while emphasising the impact upon people 65 and older, young children, pregnant women and those with medical conditions.
Closing doors, windows and blinds and using air-conditioning or electric fans can help cool homes, NSW Health said.
People in regional Australia will experience the highest temperatures on Sunday.
Port Augusta, west of Adelaide, is set for 44C while Mildura in the Victorian Mallee is due for 43C.
The Mildura Lawn Tennis Club played matches a day earlier to beat the heat, and ladies captain Brenda Hornsby was happy to relax on Sunday.
"If I can have a nice quiet day in the air conditioning, a bit of time in the pool, that will be it," she said.
The Mallee, Wimmera and large parts of rural SA also face extreme fire danger.
Lower temperatures are forecast for much of SA and Victoria on Monday but people in western Sydney have no such luck.
Bureau of Meteorology senior meteorologist Angus Hines said people in Penrith and Richmond could expect temperatures of about 39C.
Many people on Australia's west coast have enjoyed some relief this weekend following heatwave conditions last week.
People in Perth have a forecast maximum of 27C on Sunday. There were two days of extreme temperatures last week, including a record 45.1C in the Swan Valley in Perth on Thursday.
Conditions will remain hot in the north of Western Australia.
26°C