Australians sweltering through a severe heatwave were warned Thursday that the worst is yet to come, with hundreds of fires raging in several states and temperatures nearing record highs.

Most of southeast Australia has been sizzling in a heatwave that triggered a devastating wildfire on the west coast, razing 55 homes and claiming one life on Sunday.

The mercury has soared above 40 degrees Celsius (104 Fahrenheit) for successive days in South Australia and there is a total ban on recreational fires in neighbouring Victoria where players at the Australian Open fainted and vomited in blistering conditions.

Play was halted on Thursday afternoon due to the extreme heat, and a Melbourne school gardener, aged 76, died after collapsing on Wednesday in the grounds.

Commuters were urged to leave work early to ease peak-hour pressure on trains and trams, with the heat expected to cause significant delays and a wild evening hailstorm forecast.

Health officials in South Australia said 129 people had presented to hospital in the past three days with heat exhaustion and dehydration.

In Victoria state, currently experiencing its longest stretch of hot days since 1908, there were 109 cases and ambulance services reported almost double the average number of call-outs for heart attacks.

Wildfires raged across both states, with South Australia's Country Fire Service saying there were some 800 blazes of concern and an "escalating fire pattern over the next 48 hours", with total fire bans across the state from Friday.

"We may be reaching quite a dangerous phase in this heat wave and it's important that people are well aware of those risks," said state premier Jay Weatherill.

"Because we appear to be getting closer to some of the cooler weather does not mean that the threat is receding. If anything the threat is increasing."

State capital Adelaide had been forecast to eclipse its hottest-ever day -- 46.1 Celsius, recorded in January 1939 -- but it fell short with the mercury peaking at 44.2 degrees.

It was still hot enough to win the title of hottest city on Earth on Thursday for a brief period, according to the weather bureau.

Cyclists and spectators in Adelaide for next week's Tour Down Under road race were urged to be extra careful venturing into the heat, warnings that were echoed for the large crowds in Melbourne for the Australian Open.

In Victoria, 1,000 fires have been reported in the past 24 hours -- mostly due to lightning strikes -- and 39 had taken hold, with five declared emergencies.

Friday is expected to bring horrific conditions for fire crews, with strong gusts and a wind change forecast before a cool front arrives.

"It's the worst scenario possible but it's a reality," said fire commissioner Craig Lapsley. "It's forecast for tomorrow."

Officials are warning of some of the worst fire weather in the state since 2009, when the so-called Black Saturday inferno killed 173 people and razed entire towns.

A heatwave ahead of that disaster killed 374 people in Victoria and another 64 in South Australia.

The Climate Council think-tank said new research had showed that heatwaves were becoming longer and more frequent in Australia, with heat records toppling three times as often as cold records and the number of hot days more than doubling.

"Australia has always had hot weather. However, climate change is loading the dice towards more extreme hot weather," said the council's Will Steffen.

"The current heatwave follows on from a year of extreme heat, the hottest summer on record and the hottest year on record."