Heatwave conditions are forecast for South Australia, Victoria, NSW and Tasmania from Saturday to Monday.

Organisers of the Melbourne Moomba festival have been forced to cancel Monday's parade due to safety concerns, with temperatures expected to reach 38C.

"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 on Saturday.

"Parade participants are required to spend several hours outdoors in hot, heavy costumes - putting them at greater risk."

Organisers said other Moomba events would still take place along the Yarra River, where extra water stations will be set up.

A total fire ban was enacted in five Victorian districts on Saturday, with extreme fire conditions forecast to ease from Sunday.

The bureau warned the severe heat was likely to be felt across a large area including Melbourne, Ballarat, Horsham, Warrnambool and Bairnsdale.

Melbourne was tipped to reach a top of 39C on Saturday, while temperatures were set to hit 41C at Warrnambool, Torquay and Avalon.

A top temperature of 38C is forecast for Melbourne on Sunday and Monday.

The state's central district was slated to reach 41C on Saturday with temperatures of 40C forecast for holiday towns along the Murray River.

Southwest Victoria was forecast to record its sixth-highest maximum temperatures on record in some areas, with the conditions prompting the Pitch Music and Arts Festival to advise patrons to leave by Saturday morning, and those yet to arrive to stay away.

Parents and families of festivalgoers took to social media to express their concern for their children, some of whom are believed not to have reception.

"The truth is they are blocked in and can't leave if they want to," one commenter said on Facebook.

"The communication has been pathetic and they do not know the dangers."

Pitch organisers said the festival would go on and they were following advice from authorities.

"There are no active fires in the region and we wish to reassure you that those who are on site have access to ample shade, cooling misting fans, food and water," the festival said.

They also said free shuttle buses were running for those who want to leave.

Temperatures will remain hot in northern districts of Victoria until Wednesday and Thursday, when temperatures slowly drop back to the March average.

Maximum temperatures in heatwave-affected areas were expected to be 10C to 16C above the March average.

Forecasters predicted Hobart could break its minimum March temperature record of 21.1C on Sunday, while a minimum 25.6C forecast for Melbourne Olympic Park would be its highest in 11 years of operation.

The mercury was expected to reach the low 40Cs in South Australia as severe heat extended further west of the Eyre Peninsula at the weekend.

Adelaide reached 39C on Saturday, forecast to hit 39C on Sunday and 38C on Monday as the state government activated a code red response late on Friday, with additional services available to people sleeping rough.

Adelaide was in the midst of its busy festival period with heat plans triggered for events including the Fringe Festival, state athletics championship and Adelaide Cup horse race.

Harness and greyhound races have been rescheduled in many instances.

Extreme and severe heatwave conditions have also hit Tasmania, with warnings for areas including Burnie, Devonport, Launceston, Richmond, Swansea and Whitemark.

Hobart reached 34C on Saturday before a cold front brings relief, with a maximum of 25C forecast for the city on Sunday.

Southern NSW is also experiencing a severe heatwave with forecast temperatures in the high 30Cs.

Australian Associated Press