Brace yourselves Australia: there is yet another heatwave on the way. If you were beginning to think this summer was over, think again.

A pool of heat that has been brewing over inland Western Australia is about to make its way south and east as a lingering high pressure system slowly makes its way across the south of the country this week.

By mid-week heatwave conditions are expected to reach from coast to coast, with severe heatwave conditions for South Australia.

By the end of the week the heat is expected to focus on the south east and extreme heatwave conditions are expected for Canberra, eastern Victoria, south-eastern New South Wales as well as eastern Tasmania.

A heatwave is defined by the Bureau of Meteorology as "three days or more of high maximum and minimum temperatures that is unusual for that location".

Under extreme heatwave conditions even healthy people are at risk and need to moderate their behaviour accordingly.

Categorising heatwaves Heatwave severity Low-intensity The level of heat expected is unusual, but injury to people is not generally expected unless inappropriate activities are conducted or sensible precautions are not undertaken Severe Vulnerable people are at risk of injury Extreme Likely to cause impact across multiple areas such as infrastructure, transport, energy, agriculture and both healthy and vulnerable people are at risk of injury Source: Bureau of Meteorology

What's the forecast?

In the rough order of how the heat will pass from west to east:

Much of inland Western Australia has been impacted by severe heatwave conditions over the past few days. The heat will move west across the Nullarbor, while a patch of heatwave conditions is expected to remain inland of Broome for the rest of the week.

Adelaide is forecast to hit 37 degrees Celsius on Wednesday, 40C on Thursday, and 39C on Friday.

Melbourne should see the worst of the heat Friday at 41C before possible storms and 36C on Saturday.

Canberra is expected to be above 35C for the whole of the next seven-day forecast, with the heat peaking at 40C on Friday and 41C on Saturday — reaching extreme heatwave conditions for the end of the week.

It was only last week when the capital was blanketed in hail. If you kept any in the freezer it could make for handy ice packs.

Hobart is forecast to reach a maximum of 38C on Friday, with temperatures in the low 30s and high 20s in the lead-up, which constitutes an extreme heatwave for Hobart.

Most of the Tasmanian east coast is expected to experience extreme heatwave conditions for Wednesday, Thursday and Friday, with the majority of the rest of the state experiencing severe heatwave conditions.

The city of Sydney is forecast to be relatively mild, peaking with 36C on Sunday. But Penrith, just to the west, is expected to be 36C on Thursday, 40C on Friday, 44C Saturday, and 40C Sunday.

A slow moving high pressure system is forecast to move across the south of the country as a low pressure system and trough continue to bring rain to northern Australia. ( Supplied: Bureau of Meteorology )

The daytime temperatures are impressive but when it comes to heatwaves it is the prolonged heat and lack of overnight relief that makes them deadly.

Adelaide is forecast to remain above 22C between Thursday and Saturday, with a minimum of 28C on Friday.

Canberra is forecast to remain above 22C on Saturday and Sunday.

Fire danger

With the heightened temperatures the fire danger would also be expected to rise, despite the recent rain.

It is still a long way out for a fire forecast but South Australia will be the first in the firing line after the heat makes its way across the Nullarbor.

The current forecast is for severe fire danger in the Lower Eyre Peninsula, Mount Lofty Ranges, Kangaroo Island, and the Lower South East on Friday.

There may be a bit of rain in the wake of the heatwave expected over the coming days. ( Supplied: Bureau of Meteorology )

As the high moves west it is followed by a trough and frontal system, which is forecast to bring rain in a belt down from the tropics. But the current forecast is for far less rain than has been flooding the north of Queensland over the past few days.

With neutral climate drivers currently in play and with more than a month of summer still to go, this is unlikely to be the last heatwave this season.

So it is time again to: call and check in on your nan, take it easy in the sun, and keep an eye out for those around you.