Adelaide's mercury has reached a record high of 46.2C, toppling a heat record from 1939.

The Bureau of Meteorology reports that West Terrace recorded the highest temperature in 80 years at 1.42pm.

It might not be the only record that topples today.

Adelaide-based Bureau of Meteorology (BOM) forecaster Hilary Wilson told news.com.au several regions will see their records obliterated.

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"We have very hot temperatures forecast that could see quite a number of records broken," Wilson said.

The "long list of potential record-breakers" includes Clare, north of Adelaide, that previously recorded a maximum all-time temperature of 43.7C. Clare is expected to reach 45C today.

Cummins, on the Eyre Peninsula, is expected to reach 47C, breaking a previous record of 46.2C, and Roseworthy, north of the capital, could break its previous record high of 46.7C. Temperatures there will top out at 47C during midafternoon.

Adelaide was forecast to hit 45C today, but went more than a degree higher just after lunchtime.

A Sky News forecast map shows those areas turning an ominous purple colour as the severe heatwave takes hold.

Scorching temperatures were expected around Adelaide and Alice Springs - and the weather delivered. Photo / Sky News
Scorching temperatures were expected around Adelaide and Alice Springs - and the weather delivered. Photo / Sky News

The hottest temperatures are expected to be recorded at Port Augusta, 300km north of Adelaide, where it could get as hot as 49C.

Adelaide residents sweltered through uncomfortable overnight lows of just 31.1C. It was 33C at 3am.

The record-high temperatures today come with elevated fire risk and warnings are in place across the state.

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Country Fire Service state duty commander Yvette Dowling said the soaring temperatures will be unmanageable in many parts of the state.

"If a fire does start with these forecast weather conditions, we're not going to be able to control that fire," she said.

She urged the public to avoid risky behaviour in order to minimise the bushfire threat.

"The communities are at risk, our firefighters are at risk, all emergency services are at risk tomorrow."

Weather watchers from the Higgins Storm Chasing group told their 780,000 Facebook followers that conditions today were like a "blast furnace" and that "Black Saturday comes to mind when you think of this kind of heat".

"While some people may think this is just summer ... it's far from it," the group wrote.

South Australia Health chief medical officer Paddy Phillips said the heatwave will be worse than last week's, when some parts of the state recorded four days above 40C.

"In last week's heatwave we saw 69 people present to hospitals across the state with heat-related conditions and 31 of those were admitted," Phillips said.

But Wilson reckons there's relief in sight.

"For Adelaide, a southwesterly wind change will move through around midnight and temperatures should be cooler tomorrow," she said.

Karolina Pliskova of the Czech Republic takes a drink during her semifinal against Japan's Naomi Osaka at the Australian Open. Photo / AP
Karolina Pliskova of the Czech Republic takes a drink during her semifinal against Japan's Naomi Osaka at the Australian Open. Photo / AP

Meanwhile Victorians will sweat through another hot day today but it will be even hotter tomorrow.

Melbourne is forecast to reach 36C today before the mercury climbs past 40C tomorrow.

At the Australian Open, the roof to the Rod Laver Arena had to be closed early into the match between Danielle Collins and Petra Kvitova in the first semifinal.

At 2.50pm tournament officials announced the new heat stress scale tipped over the 5.0 rating needed to stop play on all courts.

They earlier came under fire for ignoring the weather forecast and opening the roof just minutes before the start of play just after 2pm.

Overnight, temperatures won't drop below 29C, making it the hottest night of the year so far, before a high of 43C tomorrow.

"The overnight minimum temperatures are nothing short of oppressive unfortunately," the BOM's Richard Russell said.

The heat in Victoria has already cut power to thousands of properties in the state's central north. More than 2700 AusNet customers were hit with a power outage near Nagambie and an underground cable fault might not be fixed until 5pm.

A cool change is expected late on Friday but it will bring with it wind gusts upwards of 80km/h and increased risk of fire. Thunderstorms are also expected.

Around Victoria, it will be even hotter. In Mildura, in northwest Victoria, temperatures are expected to reach 44C today and 46C tomorrow.

Around the country, the warm weather is continuing after the hottest December on record.

Hobart can expect a top of 27C ahead of 36C tomorrow.

In Canberra, temperatures will reach 33C today and stay hot for the next week. It will be 38C tomorrow and 39C on Australia Day.

Sydney's hottest day of the week will be Saturday, when the mercury hits 32C. Today will be 25C and tomorrow showers should clear for a top of 29C.

Brisbane's run of sunny summer days continues with highs of around 33C including on Australia Day and Darwin will reach 33C on the weekend.

Perth is experiencing dreary conditions that will clear for a run of hot weather next week.