On relative population sizes, Adelaide's 549 deaths was the highest of the five cities, with Perth and Brisbane recording mortality numbers of 532 and 220 cases of mortality, respectively.

Early season warmth on Saturday - at least in Sydney, where daytime temperatures were forecast to be climb about 7 degrees above the August average - was the latest reminder of the unseasonably mild conditions.

NSW as a whole had its warmest January-July period for daytime temperatures on record, and authorities are preparing for a busy start to the bushfire season as the intensifying drought extends to woodlands.

Dr Longden's study used a so-called Excess Heat Index developed by the Bureau of Meteorology as part of its heatwave warning service. Death rates rose markedly when heatwaves were not only hot in absolute temperatures but also compared with the previous 30 days.

"A lack of acclimatisation is an issue due to the physiological impact on people's thermoregulation," he said.

A key threshold appears to be reached when three-day average temperatures rise to more than 7 degrees above the 30-day average. Adelaide, for instance, had a spike in deaths when the index rose about 10 degrees over the January 27-30, 2009 period.

For Melbourne, the stand-out extreme case was the four consecutive days above 41 degrees on January 14-17, 2014, with temperatures 12 degrees above the preceding 30 days.

"It's about two to three days after the heatwave event when the mortality actually occurs," Dr Longden said, adding those most vulnerable often have existing health concerns, such as the elderly.

The findings point to ways authorities might improve their public health warnings in advance of a predicted heatwave. Victoria already modified their heat alert systems  after a 2009 event but more needs to be done, he said.

"There have been some public awareness campaigns...but some of the vulnerable people are the elderly who don't get those types of communications, or still get caught out by these rare and strange events that are very, very strong," Dr Longden said.

"Australians, even though we're used to hot summers, are still vulnerable," he said.

With climate change, heatwaves in Australia are predicted to become more frequent, last longer and be more intense, adding to the challenges facing all populations, including human.

"People will change their behaviour [and] have airconditioning, or try to decrease their exposure," Dr Longden said. "People who work in construction will have to stop work more often."

"How much you can offset that by behavioral changes is an open question," he said.