While there are more than 768,000 trees on public land across the ACT, the City Services Directorate only planted 1450 new street trees as part of its annual tree planting program last fiscal year.

However there is a rate of decline of about 3000 trees each year, including mature trees that are cut down and juvenile trees that either die or go missing.

Two in five suburbs in Canberra had canopy coverage of less than 20 per cent last fiscal year, while only four out of 60 suburbs had coverage of more than 30 per cent.

That data was obtained by ACT Greens crossbencher Caroline Le Couteur, who said the government needed to plant an extra 7000 trees a year to help restore the city's canopy.

She said the replacement program was "clearly failing to meet its mark" and urgent action was needed to reverse the decline.

“It’s unacceptable, at a time of rising temperatures in the bush capital, that Canberra should see numbers of trees decreasing. The last report on this issue, from seven years ago, showed that our streets and parks needed 40,000 trees to fill the gaps and replace dying trees, and unfortunately that number would be much higher now," Ms Le Couteur said.

The release of the data came after the city experienced its longest-ever run of days above 40 degrees, and as an ACT Legislative Assembly inquiry into Canberra's declining nature continues.

The Australian Institute of Landscape Architects told the inquiry there was a lack of investment in the urban forest.

"Failure to commit appropriate funding to renew and enhance Canberra’s living infrastructure is perhaps the most serious threat," the institute's submission said.

It cited a 2017 study from engineering firm AECOM, which found increasing tree canopy from 20 per cent to 28 per cent led to a four-degree reduction in air temperature and a 14-degree reduction in surface temperature.

The same study found a 10 per cent increase in tree canopy related to an average increase in property values of $50,000.

The Canberra Ornithologists Group told the same inquiry the loss of large numbers of scattered, mature eucalypt trees due to redevelopment was leading to a decline in both endangered and common bird species.

"Species such as the ACT threatened brown treecreeper have disappeared from the peri-urban and are further impacted by loss of scattered trees in the landscape," the group said.

The CSIRO told the inquiry research it had undertaken on behalf of the ACT government found Canberra's urban areas were around 8 degrees warmer at night due to pent-up heat absorbed during the day.

This effect was greatest in areas with few trees, large rooftops and lots of pavement, the CSIRO submission said.

The Inner South Canberra Community Council and the Griffith Narrabundah Council told the inquiry there needed to be on-the-spot fines for developers who failed to protect street trees during construction.

In the government's submission, Environment Minister Mick Gentleman said investing in Canberra's natural assets was the most efficient and economic way of avoiding the cost of climate change.

"With urban intensification, vegetation in the form of canopy trees and watered grass in the public realm is increasingly the city’s air conditioner," he said.

However the City Services Directorate also flagged a review of Canberra's tree protection laws as its top priority for 2018-19.

As it stands, people have to apply for permission to remove or lop trees taller than 12 metres on private land. The same rules apply for registered trees.

The laws led to a long-running court battle to remove a protected London Plane tree in Manuka as part of a multi-million dollar cinema redevelopment.