With temperatures expected to reach 40 degrees across most of the ACT for the fourth day in a row - the longest period ever recorded - the Australian Energy Market Operator has told the ACT government expected very high electicity demand had the potential to cause supply shortfalls during the hottest parts of the day.

“While the ACT has one of the most reliable electricity networks in the country, the effects of climate change, such as higher temperatures and more regular extreme weather events, are putting greater stress on our electricity grid especially during summer,” Deputy Director-General, Environment, Planning and Sustainable Development Geoffrey Rutledge said.

"We want people to stay cool ... [but] before you go to work tomorrow, turn off what you sensibly can and during the day think about whether you need to be using electrical equipment.”

The market operator said the most likely time for blackouts on Friday would be between 3-5.30pm, but said any supply shortfall was likely to be minor.

Should electricity shortfalls occur, ACT power users could face rotational load shedding, which involved cutting power to selected residential areas for up to two hours at a time.

The likely record of 40+ degree days will come amid a total fire ban for the ACT, declared by the Emergency Services Agency Commissioner on Thursday, and a severe fire danger rating.

Friday also marks 16 years since the 2003 Canberra bushfires, which saw four lives lost, hundreds of homes and thousands of hectares of bushland destroyed.

"What's causing this heat is a low pressure trough that's directing north to northwesterly winds over NSW and the ACT from central Australia," said Bureau of Meteorlogy forecaster Anita Pyne.

"[On] Tuesday, we had a maximum temperature at our observation site at Canberra Airport of 40.6 degrees.

"We [also] have a high pressure system sitting over the ocean ... which is kind of blocking other systems from coming through."

Belconnen was forecast to be the hottest place in the ACT on Thursday, with temperatures expected to reach a maximum of 42 degrees - inching closer towards Canberra's hottest day on record on February 1, 1968, when temperatures reached 42.2 degrees.

Canberra's city centre was forecast to reach 41 degrees on Thursday, and most of the ACT was expected to reach 40 degrees and above on Friday.

The nights were also expected to stay warm, only simmering down to 20 degrees on Thursday. A haze across the ACT, which was caused by dust from southern Australia, also persisted into Thursday evening.

Relief won't come until between late Friday night and early Saturday morning, when a southerly change is expected to simmer temperatures down to 33 degrees on Saturday in the city centre.

"It's much cooler, but still above the average temperature [for January] of 28 degrees," Ms Pyne said.

And, while Canberra's streets were mostly deserted on Thursday - many presumably on a self-enforced airconditioned, indoor lock down - few braved the heat as a matter of necessity or fun.

Mathew Bond and his children Cooper and Ivy were in their backyard having a splash in the sprinkler and on their slippery slide - the best way, they think, to cope with the extreme heat.

"Any opportunity to get in the water, [the kids] will," said Mr Bond, who lives in Jerrabomberra.

"They want us to take them down to Queanbeyan pool, but we can just hang around here."

Narrabundah resident Bec Lutfi and her two daughters Zahlia and Harlow had to pop out to grab a cake to celebrate Zahlia's sixth birthday. Appropriately, they chose an ice-cream cake - and an extra cone each, from Gelatissimo in Manuka, as a special treat.

"I'm 40 weeks pregnant but we're not handling the heat too badly. We've been staying in the airconditioning and eating a lot of ice-cream," Ms Lufti said.

Jack Barraclough, who's worked at the shop for about three months, said he had noticed a big increase in the number of people coming to the shop, particularly over the last few days.

"We have lines out the door of probably 25 to 30 people on Friday and Saturday nights," he said.

"We produce [the gelato], so my co-worker comes in at about 8am and spends about three hours just smashing out all of this ... it's more than we normally make."

Stuart Reed, who works for EcoWay Australia managing Canberra's local flora, said he didn't have airconditioning in his truck, so opted instead for a collection of frozen water bottles that he brought to work.

The fire danger rating is expected to decrease from "severe" on Friday to "high" on Saturday and Sunday.

The ACT Emergency Services Agency said there were no bushfires in the ACT on Thursday.

An ACT government spokesperson said there had been no increase in patient numbers at Canberra's public hospitals or in ambulances due to the heatwave.

The ACT Ambulance Service advised Canberrans to: