The City Services Directorate maintained 4327 hectares of mown grass last financial year, according to its annual report.

But the Australian National University's Fenner School of Environment and Society is concerned about the void in key habitats for native wildlife across Canberra.

As the city becomes more compact, Associate Professor Philip Gibbons says the remaining green space has to be managed in a more sustainable way - both ecologically and economically.

He says the ACT needs to look at replacing grassed areas with native landscaping, including native grasses, shrubs, dead wood, rocks and trees to improve biodiversity.

"We're not saying get rid of all grass, or that we should plant wall-to-wall of natives. But if you reduce the area of grass, the grass gets maintained better," he said.

"It costs $9 million to mow per annum, which is a fair bit of money for the ACT, and the height of grass is a constant source of complaint in the community.

"When you have high growth times around spring and summer it grows faster than the mowers can get to it, and it's dominated by two incredibly destructive weeds: Chilean needle grass, and the other is African love grass.

"I look at the park next to my place and the quality of grass is rubbish. Every ball bowled is a leg break, it goes all over the place. By reducing the amount of grass that has to be maintained, it means the grass that's left can be maintained to a better quality."

The other conundrum about all of Canberra's mown grass is that it isn't very green.

"There's been some interesting international research which shows maintaining mown grass is not carbon neutral. Grass sequesters CO2, but if you're mowing and maintaining it all the time it uses more CO2 than the grass sequesters," Associate Professor Gibbons said.

Mown grass is also "crap for biodiversity", he said, with natives species drawn to areas with more leaf litter and greater shrub cover.

"Grass areas are loved by kangaroos, rabbits and magpies but there's not a whole lot of other diversity," he said.

Having more native landscapes through Canberra could even discourage kangaroos from coming into urban areas.

"Native shrubs are not good food for kangaroos. Having more native landscapes across Canberra is not going to stop them coming to into urban areas - they're still going to get into nature strips and yards - but this might help reduce it," Associate Professor Gibbons said.

But with native plants comes higher bushfire risks.  Eighty-five per cent of houses lost to bushfires in Australia have been within 100 metres of bushland.

The solution? "Don't put native landscaping near bushland," he said.

Ecological restoration can also be done in asset protection zones in a way that can reduce fuel load, he said.

"One of the ways is to reintroduce rocks. Rocks are a great habitat for the species like the pink-tailed worm lizard. That's what they've done around Molonglo," Associate Professor Gibbons said.

"Also, large logs don't contribute to the intensity of a bushfire and can be used to reduce fuel loads in asset protection zones. It's fuels that are less than six millimetres in diameter that contribute to the intensity of a bushfire. Logs might smoulder afterwards but don't contribute to the intensity."

The ACT government has confirmed it's working with the Fenner School to trial the idea at Ginninderry, but Associate Professor Gibbons hopes it can be rolled out more widely.

However a City Services spokeswoman said mowing the urban areas was done in a "cost-effective way that meets the primary objectives of providing for safe public access, reducing fire fuel hazards and ensuring the city is well presented".

The school made the suggestion as part of an ACT Legislative Assembly inquiry into nature in the city last year.

The Legislative Assembly's environment committee is expected to hold its public hearings on the nature inquiry from March to May.

It comes as an ACT Greens MLA revealed the number of trees across Canberra is declining by 3000 a year.

City Services Minister Chris Steel said while our urban forest was ageing, the government's efforts to grow more trees were often hampered by vandalism.

"Canberra is the bush capital and while most Canberrans love our treescape, we know that there are some irresponsible people in our community who do the wrong thing. If you poison or cut down a regulated tree without approval, you are breaking the law," Mr Steel said.