The announcement on Friday by Opposition Leader Alistair Coe came as part of the party's pledge to ensure there was green space within a 10-minute walk from all Canberra homes.

Mr Coe said Labor had slashed the ACT's tree canopy from 30 per cent to 21 per cent.

The promise to plant a million trees is yet to be fully costed, but Mr Coe said the Liberals would rely on families, community groups, schools and local businesses to help deliver it.

He said each plant should cost between $10 to $20.

"A million trees will be a collaboration of the public realm as well as the private realm," he said.

"The devastating bushfire season and coronavirus has reminded us how precious our natural environment is for the health and wellbeing of local communities.

"Our long-term vision is to plant 1 million trees and preserve precious green space to ensure all Canberra families and community groups can enjoy the benefits of the fresh outdoors."

Mr Coe said the policy was based on consultation with local communities.

"Especially when you're in the Woden town centre or Gungahlin town centre, it is clear there is a shortage of green space," he said.

"People right across Canberra are desperate for more green space."

But City Services Minister Chris Steel said Mr Coe's costings were way off the actual cost of planting a million trees.

He said the $10 to $20 per tree figure given by the opposition was based on a small plant size, and did not include maintenance costs.

Mr Steel said the actual cost per tree would be more like $380.

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"Unfortunately the Canberra Liberals' plan has no detail, is completely uncosted and goes to show they're completely inexperienced with government," he said.

"$10 to $20 per tree would see a tube sock planted in an urban environment that would be trampled and simply would not reach maturity in an urban environment."

He said the government was committed to increasing the amount of green space in the city.

"ACT Labor believes green space is critically important to the character of our city," Mr Steel said.

"We have been increasing the amount of park land that is available."

Mr Steel said the government would plant tens of thousands more trees in the coming years, and wanted to increase tree canopy to 30 per cent.

Opposition environment spokeswoman Elizabeth Lee said planting a million trees would improve the overall quality of life of Canberrans.

"The most obvious benefits of trees are the role they play in carbon reduction, their shade and protection that can reduce temperatures in urban areas and of course, their natural beauty befitting our proud heritage as the bush capital," she said.

"One million trees will strengthen and protect Canberra's air quality and green spaces for generations to come."

Mr Coe has been tight-lipped about what other environmental policies the Liberals would take to the October election, but said the party would be focused on delivering "very tangible improvements to Canberra's environment".

The Liberals have already promised to freeze residential rates in their first term of government and start abolishing payroll tax if they win this year's election.