Adelaide has recorded the hottest day an Australian capital city has ever experienced, while many Victorians sweltered the hottest night in a year without power as a heatwave grips the southern states.

The South Australian capital hit 46.6 degrees at 3.35pm on Thursday, beating  Melbourne’s record of 46.4 degrees set in 2009. Another Adelaide station, at Kent Town, recorded 47.4.

But some relief is on the way,  with a cool change expected to sweep through most of South Australia and drop temperatures in Adelaide to a top of 31 degrees on Friday.

Thousands of Victorians were without power as the temperature struggled to dip below 30 degrees on Thursday night, with about 5800 properties affected by outages.

The temperature reached 40 degrees in Melbourne on Thursday and the Bureau of Meteorology has revised its forecast to 44 degrees for the city on Friday – the hottest maximum temperature since it hit 46.4 degrees on Black Saturday in 2009.

A cool change is expected about 3pm, with a temperature drop of 15 degrees in just 20 minutes forecast for Melbourne.

The sharp change will bring gusts of up to 80km/h, thunderstorms and increased fire risk, with a total fire ban in force for the state.

Towns like Mildura and Swan Hill in the north west of the state are forecast to reach 46 degrees on Friday.

Subway workers unable to keep fresh! Adelaide CBD store closed due to extreme heat. #Adelaide #heatwave pic.twitter.com/wXsjZmdJ2Y

— Dylan (@D_Schmiz) January 24, 2019

Weatherzone meteorologist Kim Westcott said it would remain hot in northern Victoria into Saturday.

South Australia had the worst of it on Thursday, when Port Augusta reached 49.1 degrees.

Ceduna set its temperature record for the second day running, at 48.6 degrees.

Port Pirie sweltered in 40.1-degree heat at 9am, and broke its heat record at 46.9 degrees later on Thursday.

Whyalla reached 48.5 degrees, while Snowtown hit 47.3 degrees and Port Lincoln got to 47.

It was 46.9 degrees in Leigh Creek and 45.7 degrees in Yorke Peninsula.

Other records smashed in South Australia on Thursday include Roseworthy at 46.9 degrees, both Edinburgh and Marree at 46.8, and Parafield at 46.6.

Stenhouse Bay, Cleve, Clare, Kuitpo and Mount Lofty also broke temperature records on Thursday.

The weather will be cooler over the Australia Day weekend, with a high of 29C expected on Saturday and 28C on Sunday, before rising to 31 degrees in Adelaide on Monday.

Just last week, parts of the state were about 49 degrees, when every state and territory endured an extreme heatwave.

Ms Westcott said it was a “pretty hard thing to go back to back” through heatwaves.

“It wears a lot of people down pretty quickly,” she told The New Daily.

“Certainly by the time we come to the end of the month there will be some big records broken for a hot January.”

SA Health on Thursday morning said 44 people had presented to emergency departments in the prior 24 hours with heat-related illness. Twelve were admitted for further treatment.

People are advised to check in with vulnerable loved ones and keep pets indoors where possible with lots of water.

Making sure the chickens don’t lay hard-boiled eggs #Adelaide #heatwave pic.twitter.com/7PL2hPdj65

— Kevin Chau (@_kevchau) January 24, 2019

Sydney is expected to top 30 degrees in the city on Friday, and reach 38 in Penrith in the western suburbs, with a total fire ban for NSW throughout the day.

Penrith is forecast to hit 42 degrees on Saturday.

Fifty fires were burning across Tasmania on Thursday afternoon, with more than half of them uncontained, as authorities braced for worsening conditions.

“On a whole, across the whole part of the state, I haven’t seen conditions at this level,” Tasmania Fire Service regional chief Jeff Harper said on Thursday.

Conditions were set to peak from 5am on Friday, with hours of dry air, temperatures reaching into the high 30s, and 40km/h winds gusting up to 90km/h.

Two fires have the potential to threaten communities. One is a blaze at central Great Pine Tier, which could bear down on the township of Shannon, and the other is the Gell River fire south-west of Hobart, which has been burning since December 28.

Residents are being told of the impending threat while efforts are being made to clear fuel from around the communities.

One house has been lost near central Miena. Residents remain on alert at Barren Tier, Tods Corner, Penstock Lagoon, Liawenee and Shannon.

More than 2700 AusNet customers were without power in north-central Victoria near Nagambie on Thursday, where it was forecast to hit 41 degrees.

Power was cut because of a fault in an underground cable. Crews were digging to fix it.

-with AAP