Melbourne and Hobart sizzled overnight, with the Bureau of Meteorology recording unprecedented temperatures for both cities.
"There is going to be a lot of tired and cranky people across southeastern Australia," meteorologist Sarah Scully said on Sunday.
The overnight temperature in Melbourne brought little reprieve from the sweltering heat. (Diego Fedele/AAP PHOTOS)
Melbourne sweated as the mercury hovered near 30C for most of Saturday night before dropping to 27.3C early on Sunday, toppling the previous March record high minimum temperature of 26.5C in 2013.
Hobart was also uncomfortably hot overnight, with a minimum temperature of 24C, well above the previous March record of 21.1C, Ms Scully said.
Authorities issued a total fire ban for parts of South Australia on Sunday, as the mercury soared into the high 30s and 40s across most of the state.
The bureau has warned the heatwave conditions will persist into next week in parts of NSW, Tasmania and SA, and most of Victoria, where a total fire ban has been issued for the state's southwest on Monday.
Melbourne peaked at 36.9C late on Sunday, with Avalon recording 40C and Geelong 39.6C.
Temperatures were much cooler in Tasmania as a cold front pushed across the state, with Hobart's maximum temperature of 25.7C recorded before 8AM.
Extreme conditions earlier in the weekend forced event organisers to cancel come outdoor festivals and parades.
Melbourne's famous Moomba Parade was cancelled on Saturday due safety concerns for performers and spectators.
"This is a very difficult decision, particularly in Moomba's 70th year, but we must prioritise people's health in these extreme conditions," Melbourne Lord Mayor Sally Capp said in a statement.
"Parade participants are required to spend several hours outdoors in hot, heavy costumes - putting them at greater risk."
Patrons at the Pitch Music and Arts Festival in southwest Victoria were told to leave by Saturday morning as those yet to arrive were warned to stay away.
"If you are arriving on Saturday we recommend delaying your arrival until further notice," the Pitch Instagram post read.
"The forecast tells us to expect hot weather each day of the festival. In addition, overnights will also be warm."
One of the stages at Adelaide's WOMADwas closed on Sunday due to the heat, while a handful of other events were postponed till late at night or cancelled as temperatures climbed to almost 40C.
Mist generated by hoses suspended in the air brought some relief to hot patrons at WOMADelaide. (HANDOUT/WOMADelaide)
Of 71 people who sought first aid from St John Ambulance, more than half had minor complaints related to the heat, while crowds gathered around the mist generated by hoses suspended in the air.
Melbourne is tipped to reach a maximum of 37C on Monday, with temperatures in the mid to high 30s predicted across most of the state.
Adelaide is expected to climb to 38C, with hot conditions in the high 30s and low 40s predicted for most regional areas.
23°C