And the Bureau of Meteorology has indicated it's going to get hotter, with highs of 39 degrees on Wednesday and Thursday, although Monday is forecast to be a relatively cool 33 degrees.

On Tuesday, the anticipated sell-out crowd of about 12,500 at Manuka Oval for the Prime Minister's XI cricket match is set to endure 37-degree heat.

Coastal areas will be spared, with sea breezes keeping temperatures down; a top of 33 degrees is forecast for Batemans Bay this week.

Locally the heatwave is likely to see plenty of mid-week activity at local pools and on the river, with temperatures in the high 30s persisting until Sunday.

Tristan Viscarra Rossel and her family were among the swimmers at Casuarina Sands on Sunday, and she said she would return during the week to keep cool.

The family usually swims at Kambah Pools but decided to try a new spot. ''It's more accessible for our three-year-old - walking up and down the hill at Kambah Pools is quite difficult with a little one, so that was good, but it was a lot more crowded,'' she said.

With a fire-danger rating of very high on Monday, Green Cross Australia is asking Canberra residents to take the time to plan for major weather events.

Using federal and ACT government funding, the not-for-profit group has developed the website actfirst.org.au to help residents prepare for severe weather such as fires and heatwaves.

''Some key actions people can take [in a heatwave] include staying up to date with local news and weather safety updates, drinking plenty of water, focusing on keeping just one room in their house cool … and keeping an eye on people who are vulnerable to heat, such as elderly neighbours and young children,'' Elizabeth Resta from Green Cross Australia said.

The ACT is not the only region experiencing the heat, either.

The Bureau of Meteorology is testing a Heatwave Forecast tool, which shows that much of southern Australia will experience the effects of the heatwave during the next four days.

''It is caused by slow-moving, high-pressure systems and, basically … we're predicting hot air from central Australia just building up that heat,'' the bureau's duty forecaster said.

Melbourne will be hit particularly hard with an ''extreme heatwave'' forecast to greet the start of the Australian Open tennis.

Tuesday and Thursday will be the hottest days with 41 degrees forecast for both, and 40 predicted for Friday.

The players won't be able to get any quick relief from the dehydrating conditions either. Tournament medical staff will not administer saline drips, as they have during extreme heat in the past. The only exceptions will be for emergencies.

Australian Open chief medical officer Tim Wood said World Anti-Doping Agency regulations banned players from being placed on a drip after a match.

''If they want a drip, they have to go to hospital,'' he said.

To improve their performance in the heat, some players take an ice bath before a match. Dr Wood said this could reduce the core body temperature by as much as 1 degree.

Australian Sports Commission exercise physiologist Megan Ross said ''pre-cooling'' made a difference, prolonging the point of fatigue.

with Bridie Smith