Adelaide has hit a sweltering 46.6 degrees Celsius, surpassing the previous record set in Melbourne a decade ago to officially become the hottest capital in the country.

Key points: More than 20 locations hit record temperatures in South Australia on Thursday

More than 20 locations hit record temperatures in South Australia on Thursday Meteorologist Hilary Wilson said records had been broken right across the state

Meteorologist Hilary Wilson said records had been broken right across the state The CFS warned communities were at risk during extreme heat

Baby Emilia enjoys a cool bath in her Port Willunga backyard. ( Instagram: @casamaculata )

Earlier on Thursday, Adelaide topped its 1939 heat record after hitting 46.2C.

The temperature then climbed up to 46.6C, topping the previous capital city record of 46.4C set in Melbourne in 2009.

More than 20 locations hit record temperatures in South Australia on Thursday, including Adelaide Airport, Minlaton, Noarlunga, Snowtown and Port Lincoln.

Ceduna set a record for the second day in a row, reaching 48.6C.

Meteorologist Hilary Wilson said records had been broken right across the state.

"It really is quite something to see this many records broken in one day," Ms Wilson said.

"It's preliminary at this stage but when we do the figures it wouldn't be at all surprising to see that this was the hottest day ever for South Australia as a whole."

Temperature records tumble

People cool off in the water at the Port Lincoln jetty. ( ABC News: Samantha Jonscher )

Earlier in the day, the BOM said up to 23 locations in South Australia could break record high temperatures, including Mount Barker, Clare and Maitland.

The Yorke Peninsula town of Minlaton was the first town to beat its previous record of 45.6C.

Records broken at: Adelaide

Adelaide Adelaide Airport

Adelaide Airport Clare

Clare Edinburgh

Edinburgh Hindmarsh Island

Hindmarsh Island Kuitpo

Kuitpo Marree

Marree Minlaton

Minlaton Minnipa

Minnipa Noarlunga

Noarlunga Parafield

Parafield Port Lincoln

Port Lincoln Port Pirie

Port Pirie Roseworthy

Roseworthy Snowtown

Snowtown Stenhouse Bay

A temperature of 45.7C was measured by the BOM at 11:14am.

Adelaide's previous maximum temperature of 46.1C was set in 1939, just above the original 45C Thursday forecast.

"Port Augusta actually set a new record last week of 49C, it's not quite there yet, but it's certainly very close," Ms Wilson said.

At 4:11pm Port Augusta hit 49.5C.

"Elsewhere Port Pirie has exceeded its previous record of 46.6C, it reached 47.8C so that's another significant record," she said.

Whyalla also exceeded its record temperature, reaching 48.5C, while Leigh Creek has a new record of 46.9C.

She said while forecasters generally predicted the weather for the next week, it had been demonstrated under climate change scenarios that these type of extreme events were more likely to occur.

"I don't think [another record] will be anytime soon but we might not have to wait another 80 years either."

She said a wind change was expected to reach Adelaide about midnight, although little rain was expected with the change.

SA Health said 44 people had presented to hospital emergency departments with heat-related illnesses in the past 24 hours.

It said 12 people were admitted for further treatment.

The heatwave also prompted fire bans in 13 districts.

On Wednesday, SA Country Fire Services (CFS) state duty commander Yvette Dowling warned communities were at risk in the extreme heat.

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

An ice delivery man makes the most of the leftovers in Orroroo. ( ABC News )

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

Adelaide Metro said that due to the extreme heat it would run trams "at [a] reduced frequency" between the Adelaide Entertainment Centre, west of the CBD, and the Adelaide Botanic Gardens.

Baking in the heat

Rose and Patrick Wynne, from Echunga in the Adelaide Hills, cooked a batch of sun-baked peanut cookies in Wednesday's 40C heat.

Mrs Wynne said the couple were intrigued by stories about people cooking things in their cars and outside in the heat and used an old tabletop oven with a microwave plate.

"We put it out about 12:00pm and I left it until 4:00pm and they just slowly baked. They were perfect. They snapped crisply and were perfectly done," Mrs Wynne said.

"They weren't like soft or mushy. They were just perfect.

"They were probably done earlier than that [but] we left them out to make sure. They didn't go brown on top, it is not an intense heat."

Sorry, this video has expired A couple baked biscuits in the Adelaide Hills.

She estimated the temperature in the box was between 80C and 100C.

"We heard about it before, people baking cookies in their cars and all that sort of thing," she said.

"We're a bit… power saving nuts anyway so we thought we might try it.

"You don't get a hot kitchen and [you get] cookies at the end of the day."

'Incredibly high' temperatures overnight for Victoria

BOM Victoria's Richard Russell said temperatures in parts of the state were going to be "nothing short of oppressive", particularly in northern Victoria where it was expected to reach the mid-40s.

Mildura was forecast to reach 44C, Bendigo 41C, Wangaratta 41C, Warrnambool 39C and Melbourne 36C.

"We'll see those conditions spread across most of the state for Friday [with] incredibly high minimum temperatures on Thursday night… in Melbourne we expect the minimum temperature to only get down to 29C, much of the state will barely dip below 30C during that period," he said.

"Melbourne's all-time overnight record is 30.6C, so it's getting pretty close."

The heat proved harder for some, with about 2,700 houses in the central Victorian towns of Nagambie and Avenel left to battle an anticipated maximum of 41C with a power blackout.

AusNet said a fault with an underground powerline would take several hours to repair.

The power outage caused the closure of the public pools in Nagambie and Avenel, and the local council set up a relief centre to help people escape the heat.

The power company said crews were working to install a generator in Nagambie, but it was expected to only service 400 customers.