Health warnings have been issued as a scorching heatwave starts to move from Western Australia into the country's south-east.
Authorities are warning temperatures will be in the 40s throughout the week and could cause sickness and even death among the very young and elderly.
Doctors, nurses and paramedics are preparing for a spike in demand and residents are being told to stay hydrated and as cool as possible.
The risk of fire in many areas will also be extreme.
The Bureau of Meteorology's Alisdair Hainsworth says Western Australia has just ended a sweltering week, with temperatures of up to 48 degrees.
"That really extreme heat is now moving eastwards and is expected to move into South Australia and north-west Victoria over the next 24 to 48 hours," he said.
"Then it will gradually move down across the rest of south-eastern Australia, into southern New South Wales and western New South Wales as the week progresses.
"So really 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."
In South Australia, where temperatures above 40 are expected until Friday, the weather bureau has issued an extreme heat warning from today until Thursday.
The State Emergency Service is co-ordinating the response.
"Heat has a huge impact on the human body," acting deputy chief Scott Turner said.
"A couple of degrees affects the human body a lot more than anything else."
Mr Turner says people need to look after themselves.
"Statistically we've seen every year when we've had hot and protracted summers, the mortality rate goes up, presentations to hospital go through the roof and our ambulance services come under extraordinary demand," he said.
The Red Cross will be phoning at-risk residents three times a day to check they are all right.
There will be extended trading hours on some government-run facilities, like the Adelaide bus station and public swimming pools.
Many businesses also say they will make their facilities available for the public.
© ABC 2014
17:34 EST Drought-breaking rain has brought long-awaited relief to some farmers in western New South Wales.