A heatwave of temperatures in the mid-to-high 30s was tipped to swelter the capital until Friday.
Tops of 38 degrees on Wednesday and 36 on Tuesday and Thursday were expected to follow Monday's maximum of 35.
2015 hottest year on record
Video shows mass coral bleaching of Great Barrier Reef
Shark attacks: Australia's most dangerous beaches
Coastal erosion: a sign of things to come?
Victoria receives 'significant' bushfire warning
Snake breeding season slithers into Sydney
New dinosaur discovered in Queensland
Smoke blankets Sydney's skyline
2015 hottest year on record
The Bureau of Meteorology releases its annual climate report revealing Australia has had one of its hottest and driest years on record in 2015.
Overnight temperatures were likely to be 5 to 6 degrees above average, with a minimum of 21 degrees on Thursday.
But Canberrans will score quick relief from the hot spell, with a significant 15-degree drop set for Friday.
They may also see up to 40 millimetres of rain across Thursday and Friday, which is more than half Canberra's average monthly rainfall.
Bureau of Meteorology climatologist Acacia Pepler said about one in four years has a string of days reaching 35 degrees or higher.
"There have been 20 heatwaves since Canberra Airport opened in 1939, of which 16 were in January," she said.
"The longest spell was nine days of 35 plus, over 21-29 January, 1947.So this heatwave is significant, but not record-breaking."
However, it is much hotter than last year's "relatively cold and wet" January, which only had one day above 35 degrees.
This summer would have likely had five days above 35 degrees after this heatwave, while the average year has three days that hot.
The hottest January day on record for Canberra was 42 degrees on January 18, in 2013, and the hottest day for any month was 42.2 degrees on February 1, in 1968, Ms Pepler said.
While this week's temperatures are well above January's average daytime temperature of 28, the expected rainfall was also high.
Thursday and Friday could get more than the 35 millimetres that fell throughout December.
The cool change will see a top of 21 on Friday, 23 on Saturday and 27 on Sunday, with temperatures likely to stay in the high 20s for most of the following week.
The sudden drop in temperatures is caused by a deep low pressure trough followed by a cold front, Weatherzone meteorologist Kim Westcott said.
"We are going to see much cooler air moving behind this cold front," she said.
"You'll also notice that Friday will be showery, cloudy and a bit windy at times, too."
But until the heat subsides, ACT Health advises Canberrans to keep hydrated, stay rested and eat fresh food.
People should limit non-essential strenuous activity, avoid alcohol and caffeine when dehydrated and take cool showers.
"Wear lightweight clothing and be sun-smart," the website says. "Check on others including children, elderly, people with medical conditions and don't forget your pets.
"Never leave anyone (particularly children or the elderly) in an unattended car. Temperatures inside a car can rise to 50 degrees within a few minutes, leading to serious illness or death."
Children's safety is said to be a priority for John Skene, who regularly photographs kids swimming during the summer.
He recently entered The Canberra Times' summer photocomp with a picture of his six-year-old family friend Callan Dunstan enjoying a waterslide at a Christmas party.
"I'm well aware that little ones can get into trouble in six inches of water," he said.
"I'm firstly conscious of safety, then it's also about fun and catching the moment."
To enter this season's competition, send in a maximum of three photos with a summer theme to email@example.com as attached JPEG files at least 150 kilobytes and not more than one megabyte in size. Include your name, address, phone number, photo title, a description of the photo and the date it was taken.
Click or touch here for full terms and conditions. The competition ends 12.00am February 28