Perth sweltered through a heatwave over the weekend with temperatures reaching 44 degrees Celsius (111 Fahrenheit).
That heat system is now moving east across the continent, with temperatures in excess of 40 degrees forecast in Adelaide and Melbourne this week.
Hospitals and emergency services are on standby with forecasters warning of a "fairly extreme" heat event.
"We're really looking at a fairly extreme heatwave where temperatures are going to reach into the 40s for quite a number of days in a row," a bureau of meteorology spokesman said.
Some swimming pools, shopping centres and public libraries will offer extended opening hours and the Red Cross will make regular calls to at-risk residents, particularly the sick and elderly, to check on their welfare.
South Australia state's health department urged residents to stay indoors, wear cool clothing and drink plenty of water.
Wildfires are a common feature of Australia's December-February summer months, with a devastating firestorm in 2009 killing 173 people and razing thousands of homes in south-eastern Victoria state.
Unseasonably early infernos broke out in the Blue Mountains west of Sydney in October, destroying more than 200 homes and claiming two lives.
That followed an unusually dry and warm winter and record spring temperatures, prompting debate about the role of climate change.
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