After breaking up from school, 13 year old Xavier Graham from Lyons, cools off in Lake Burley Griffin with a dip and a game of catch with a mate, Jackson McLean today.

After breaking up from school, 13 year old Xavier Graham from Lyons, cools off in Lake Burley Griffin with a dip and a game of catch with a mate, Jackson McLean today. Photo: Graham Tidy

Canberra is set to swelter through its hottest nights since last summer, with the city's nighttime lows staying above 17 degrees on Thursday and Friday.

Heading for a top of 35 degrees and mostly sunny conditions on Thursday, the temperature went past 34 degrees at 2.30pm and was still above 34 two hours later.

Weatherzone meteorologist Ben McBurney said the heat would continue through the afternoon.

Out for temperatures January-March

Out for temperatures January-March

"The temperature is going to stay on the rise in Canberra, so its still on track for the forecast top of 35," he said.

"Tomorrow is expected to be even hotter again, with an expected top of 37 degrees."

Mr McBurney said Canberra's December average top was 26.1 degrees.

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"For Canberra standards, it's going to be quite a warm evening. We're heading for lows of 17 degrees ... which will be the hottest night since last summer."

"Friday night into Saturday is also going to be a very warm night when it could get down to 18 degrees."

He said Saturday would bring a weak cool change and a top of 32 degrees, before cloudy conditions and 34 on Sunday.

Canberra High year 10 students Jeremy Bradbury and Triska Hoelzl  escape the heat at the Big Splash water park with water filled balloons last month.

Canberra High year 10 students Jeremy Bradbury and Triska Hoelzl escape the heat at the Big Splash water park with water filled balloons last month. Photo: Jay Cronan

Monday's forecast top is 31 degrees with possible showers.

"Christmas Day on Tuesday is looking like a pretty good day [for Canberra] with mostly sunny skies and a top of about 26 degrees at this stage," Mr McBurney said.

The Bureau of Meteorology said on Wednesday that Australia is in the grip of its first heatwave of the summer, warning holidaymakers to stay hydrated and avoid prolonged exposure to the sun.

The slow-moving high-pressure system that brought blistering heat to Perth during this week's Ashes Test is now sending the mercury into the high 30s or low 40s across much of the continent.

Melbourne has reached 39.3 degrees on its way to a top of 40, while Sydney is sitting on 25.3 degrees with a forecast maximum of 28.

The ACT Rural Fire Service and ACT Fire and Rescue have extinguished a two hectare grassfire fire burning at Paddys River.

An Emergency Services Agency spokeperson said seven fire units were conducting clean up operations near the river at Point Hut Crossing.