Australia heatwave forecast with temperatures to top 40C

It is still a few days out from the official start of summer, but in Australia the heat is set to ramp up this week and residents are being urged to heed health warnings.

Severe heatwave conditions are set to stretch across the continent - from north-west to the south-east - as heat is drawn down from the centre.

Tracts of the country face severe heatwave conditions and this will not be a one-day affair.

To hit the official criteria for a heatwave, the Bureau of Meteorology (BOM) requires "three days or more of high maximum and minimum temperatures that are unusual for that location" - the heat is expected to hang around all the way into next week.

The heat has been building over northern and central Australia for weeks now and, according to BOM senior forecaster Jonathan How, a cold front is set to push that heat into the south-east.

As of Wednesday, temperatures will heat up in South Australia and western Victoria and hit the high 30s.

From Thursday, the heat is expected to build and extend further over inland New South Wales.

By Friday, temperatures are expected to be 10 to 20 degrees above average for southern South Australia and western Victoria and reach the high 30s and low 40s.

Saturday is when the heat is expected to peak, bringing the first real blast of prolonged heat to the south-east - temperatures are expected to be up there with November records.

"At this stage, 46C to 47C is looking like some of the high temperatures we will see on Saturday around central and eastern parts of South Australia," How said.

Sea breezes are likely to keep things cooler over Melbourne, but Adelaide is set to be the hottest capital, reaching 40C on Friday and Saturday.

On Sunday, the heat is set to move into coastal New South Wales, including Sydney, where the city's west is forecast to reach the low 40s.

The heat is expected to dissipate a little by Monday, but heatwave conditions are expected to move up into northern New South Wales and south-east Queensland.