Sydney and Melbourne have been warned to prepare for scorcher days reaching 50°C by the end of the century - even if global warming is contained to the Paris Agreement target of a 2° increase.
A new study led by Australian National University (ANU) climate scientist Dr Sophie Lewis has projected daily temperatures 3.8° above existing records for the two cities and even hotter extremes.
"We have to be thinking now about how we can be prepared for large population groups commuting to and from the CBD on these extremely hot days, how we send young children to school on 50° days, how our hospitals are prepared for a larger number of admissions of young or old people, and how our infrastructure can cope with it," Dr Lewis said.
The study found containing global warming to 1.5° - the more ambitious target set by the Paris Agreement - would limit extreme heat, but Dr Lewis said angrier summers were inevitable.
"We've already seen an increase in excess heat deaths in heatwaves in 2009, due to those extreme heatwaves, and that's likely to occur even more under these 50° days."
Pockets of Australia have tasted temperatures close to 50°, mostly remote country towns.
But Dr Lewis said heat like that would look very different in Sydney or Melbourne.
The ANU study only analysed Bureau of Meteorology data from Sydney and Melbourne, but Dr Lewis said all of Australia could expect to see hotter extremes in the future.
"It's not great news, obviously," Deputy Lord Mayor of Sydney Jess Miller said.
Ms Miller said one of the issues facing Sydney and Melbourne was the emergence of "heat continents", where entire suburbs baked for a significant period of time without relief.
"When you've got grey infrastructure and roads and buildings absorbing all that heat, not only does it get much hotter, but it takes twice to three times as long to cool down," she said.
And Ms Miller said 50° days also posed a threat to city transport.
All Sydneysiders know the sense of dread as they pack into Central Station on a sweaty day, but Ms Miller said it was not only the commuters who would suffer if the temperature reached 50°.
"When you have a bunch of days one by one, it stops the whole system."
Ms Miller said city planners needed to begin designing cities that took advantage of wind, green spaces and shade whilst still being cost-effective.
"We need to think of ourselves as part of the ecology of a city, and that a city is not just a bunch of buildings and roads," she said.