Parts of NSW, Victoria and South Australia are facing periods of 12C above the March average during the sweltering weekend.
Melbourne raced into the mid 30s on Saturday while Adelaide peaked just short of 40 degrees and Sydney is bracing for a maximum of 37C on Sunday.
It's hotter still inland, with Mildura in Victoria's northwest hitting 41C and South Australia's Renmark 42C on Saturday.
The heat is expected to stick around into the evening with the weather bureau predicting record-breaking March minimums overnight, especially across northern parts of Victoria including Shepparton and Albury-Wodonga.
"It is quite late in the season to see the heat (but) not unprecedented," Bureau of Meteorology senior forecaster Jonathan How said.
"What it means is that we will see the temperatures up to 12 degrees above the March average and elevated fire dangers as well."
Mr How said the bureau had seen elevated sea surface temperatures for months.
"So it's still quite pleasant in the water but still very much above average for this time of year," he said.
"So we are reminding people if you head to the beach to escape the heat, check for the latest wind and marine warnings."
Authorities are also warning the hot, dry conditions could increase the risk of bushfires in all three states.
NSW Ambulance Inspector Andrew Bibby described the heatwave as a silent killer.
"In Australia, more people die from heatwaves and extreme heat illness conditions than they do from floods, fires and all other natural disasters combined," he told reporters on Saturday.
"This weekend is not a weekend for heavy physical activity."
South Australian Country Fire Service chief officer Brett Loughlin said the conditions were perfect to fuel fires.
"We're going to see those high temperatures combine with strong winds and this record dry period that we find ourselves in across our state to create all the ingredients that we need for fire to spread rapidly across the landscape and to be a threat to communities."
The swelter is expected to give way to a cold front moving northeast.
"Thankfully, it's not a long sort of heatwave," Mr How said. "We will see relief coming through in the next 24 to 36 hours."
South Australia and Victoria are likely to feel the reprieve on Sunday but NSW won't see temperatures drop until Monday.
The cold front will likely bring widespread rain and storms.
Southern Queensland will also enjoy dry, calmer weather allowing floodwaters to recede and recovery efforts from ex-Tropical Cyclone Alfred to continue.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
