It also comes after a hot night, when it was still 22.6 degrees at midnight. The temperature stayed above 20 degrees almost all of Saturday night and Sunday morning, briefly dipping to 19.8 and 19.9 degrees between 5.30am and 6.30am before the mercury began to rise again.

The Bureau of Meteorology forecasts an 80 per cent chance of rain to round out a stifling weekend, with a relatively balmy high of 32 degrees. However, it only expects 1mm to 4mm of rain, most likely during the afternoon and clearing during the evening. There is also the chance of a thunderstorm on Sunday morning.

There were several lightning strikes during Saturday night's thunderstorm, and there were reports of smoke near the NSW border, however, a helicopter flight over the area did not find any fire.

The slight relief will end two days of maximum temperatures above 40 degrees, the third time such a string has happened in Canberra since records began.

Canberra Airport's weather station recorded a 41.1 degree peak at 2pm on Saturday, up from the 40.4 maximum on Friday.

That was not enough to beat the 42.2 degrees on Canberra's hottest recorded day on February 1, 1968.

Emergency Services were kept busy on Sunday racing out to a grass fire in Weston shortly after 1pm which grew to 20sqm in size before it was extinguished.

Smoke streaming out of Weetangera Primary School on Southwell Street had passersby worried, however ACT fire crews arrived before 2pm to find there was no fire and the smoke had been caused by an electrical short.

They remained on scene to monitor the school until electricians arrived.

Across the border, NSW faced the "worst possible fire conditions" in its history with a "catastrophic" warning in place across large slabs of the state after a heatwave smashed temperature records on Saturday.

Rural Fire Service Commissioner Shane Fitzsimmons said the situation was as "bad as it gets" and warned it was set to get worse on Sunday when winds are expected to sweep through scorched parts of mid to northern NSW.

"To put it simply [the conditions] are off the old scale," he said. "It is without precedent in NSW".

Commissioner Fitzsimmons said "catastrophic" fire ratings had been issued only once before in NSW - in 2013 - since national standardised ratings were introduced in 2009. Sunday's catastrophic fire rating will stretch from Dubbo to Coonabarabran to Port Stephens, affecting the Central Ranges, North Western NSW and the Greater Hunter.

"This is an area three to five times larger than January 2013," he said, when more than 140 fires were burnt across the state.

"[Any fire] will consume whatever is in its path."

On Saturday afternoon, the RFS said 49 bushfires or grassfires were already burning across NSW, 17 of which were not contained but no loss of life or property had been recorded.

In the ACT, firefighters tended to at least two grass fires on Saturday, and a fiery car crash in the Parkes Way tunnel, all of which were extinguished quickly.

A total fire ban across the territory will remain in place until 11.59pm on Sunday.

Conditions are expected to ease from Monday to Wednesday.

Power problems plagued the capital on Saturday, with outages in parts of Monash, Wanniassa and Gungahlin. Residents have been asked to continue keeping their power usage to a minimum.

There was good news for bookworms, however, as Belconnen Library fixed the air conditioning problem that forced its closure on Friday afternoon. The library will open as usual on Sunday.

The forecast temperature drop will come as a relief to the city's wildlife as well. At Tidbinbilla, rangers cooled off the native inhabitants with buckets of water.

National Zoo and Aquarium operations manager Renee Osterloh said their animal handlers provided a range of icy treats in the hot weather.

She said most of the animals slowed down and sought cool places to conserve energy and rest during hot summer days.

"We've got bloodsicles, fishicles and fruitsicles," she said.

Temperatures are expected to rise again before the weekend, however, with forecast highs of 34 degrees on both Thursday and Friday.

- with Eryk Bagshaw, Megan Levy, Peter Hannam