Studies have shown that the suburb of Macgregor was up to 7 degrees hotter than certain southside suburbs. L-R Aideen Fitzgerlad,9,  cools off with Nate Fitzgerald,2,  and Seth Fitzgerald,5  in their backyard in Macgregor.

Studies have shown that the suburb of Macgregor was up to 7 degrees hotter than certain southside suburbs. L-R Aideen Fitzgerlad,9, cools off with Nate Fitzgerald,2, and Seth Fitzgerald,5 in their backyard in Macgregor. Photo: Melissa Adams

As Canberra sweltered through its hottest day in 12 months, Australian National University data suggested trees could be the key to lowering summer temperatures by up to seven degrees in the capital's suburbs - and saving lives.

The study of temperatures and humidity found the outer suburb of Macgregor hit a maximum of 42.5 degrees on summer's hottest day four years ago, 7.5 degrees warmer than in the leafy inner south at Deakin.

On Wednesday the thermometer hit 40.2 degrees at 3.25pm, heat not seen since January 18 last year.

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ANU academic Liz Hanna said the 22-day study of 21 suburbs - which found consistent higher temperatures in outer areas compared to inner suburbs on either side of Lake Burley Griffin - confirmed international studies which linked temperature variation to urban design and foliage.

''Green spaces and foliage cool the environment and lessen the health risk, especially on these very hot days,'' Dr Hanna said.

''The striking difference in Canberra is likely to be based on the thick canopy of mature, shady, deciduous street trees.''

The unpublished data was part of background research done for an upcoming study on working in the heat in Australia to be completed by ANU's National Centre for Epidemiology and Population Health.

Dr Hanna, the director of the Working in the Heat project, said the temperature findings highlighted the need for larger front and backyards.

''If we have planning designs and housing designs where there's insufficient front and backyard to plant a tree, and if street trees are not shade trees, we think that's foolish,'' she said.

''It actually makes a difference because when it gets to certain temperatures, people start flocking to hospitals and morgues, and it's a relatively easy step we can do to ensure there are good shade trees planted, and that they're looked after in the heat as well.''

Each year, a number of Canberrans are affected by hot temperatures leading to hospitalisation, and in some cases death.

Click or touch on the icons above to see the temperature at 3pm on January 12, 2010.

In Macgregor, the Fitzgerald family know all about the heat left by a lack of trees both on and surrounding a block.

While the three kids were happy to have a quick water fight in the small backyard at 6.30pm on Wednesday, Paul Fitzgerald said the lack of trees and a residential construction site across the road created a baking dustbowl on hot days. ''Out here they just fry,'' Mr Fitzgerald said.

''We'll be off to the pool tomorrow. We get some massive winds, and we go to the mall, because it just gets so stuffy [inside].''

He and his wife Zarina blame the planning and development of small blocks, with their recently built rental home covering between 350 and 400 square metres.

''We're hemmed in like sardines here just so people can make more money,'' Mr Fitzgerald said.

In ''old'' Macgregor, just a short drive to the east, the Esterhuizen family have established trees just outside the front yard, with an extensive covered patio providing wider relief from the sun. ''There was nothing in [our previous home in] Franklin, but here there's some shade from trees and we have the deck,'' Johan Esterhuizen said.

Mr Esterhuizen said the family migrated from South Africa in 2009, with the airconditioner used more frequently in summer at both their Canberra dwellings.

''In South Africa we had airconditioning in our house, but we never used it as much as here,'' he said.

Environment and Sustainable Development Directorate director-general, Dorte Ekelund, said planning controls for development on suburban blocks did not limit or prevent factoring in trees. ''The ACT is committed to protecting hills, ridges and watercourses, providing urban parks and ensuring street verges are treed which all contribute to the 'bush capital','' Ms Ekelund said.

''Often new suburbs are more exposed to wind, dust and hotter temperatures while the vegetation establishes.''

While Dr Hanna encouraged residents to water their nearby trees, Canberrans were already making major use of the liquid resource, with ACTEW Water reporting the city's 250 megalitres used on Wednesday as the largest consumption since Spring 2006.

There's no temperature relief for Canberrans in any suburb tomorrow, with another 40 degree maximum tipped, followed by top temperatures of 39 on Friday and Saturday.

Thursday is a day of total fire ban in the ACT and the Southern Ranges, Central Ranges and Southern Slopes regions of NSW. Isolated showers are expected from Saturday afternoon, with a milder 30 degrees the peak on Sunday.

Dr Hanna is calling for people who work in extreme heat conditions to take part in this year's study, and would like to hear from individuals and employers.

Those interested in taking part can email summerheat@anu.edu.au. For more information visit nceph.anu.edu.au/research/projects/working-heat-study