Chief Minister Andrew Barr made the promise to preserve the sprawling grasslands during a debate over Canberra's rental prices in the ACT Legislative Assembly on Thursday.

Mr Barr also used the speech the reaffirm a previous promise to protect the Kowen Forest pine plantation east of Canberra from redevelopment.

The move to rule out housing west of Tuggeranong doesn't represent a significant shift in policy from the Labor government.

Mr Barr expressed "deep" concern about the possible expansion of housing west of the Murrumbidgee after the National Capital Plan was amended to allow it in 2016. The government also removed West Murrumbidgee from consideration in its 2018 planning strategy amid environmental concerns.

The commitment on Thursday was an attempt by Labor to wedge the Liberals, who have promised to boost land supply if elected later this year but are yet to outline in what areas.

Liberal senator Zed Seselja has strongly advocated for housing west of the Murrumbidgee, even accusing Mr Barr of "selling out Tuggeranong" when he rejected the idea in 2016. Opposition leader Alistair Coe last year said Kowen Forest should remain an option for future residential development.

Labor's pledge was incorporated into a motion in the Legislative Assembly, which the Liberals opposed.

Their position prompted Planning Minister Mick Gentleman to claim the grasslands and Kowen Forests were now under threat.

"The Canberra Liberals owe it to Canberrans to be upfront about their development plans and what the environmental impacts will be," Mr Gentleman said.

Asked where the party stood on either preserving or redeveloping the land in Tuggeranong and Kowen, a Canberra Liberals spokeswoman said: "We will protect land that is ecologically significant".

The spokeswoman repeated the Liberals' commitment to plant one million trees and guarantee a park within a 10-minute walk of every Canberra home if they win government.

"Labor and the Greens have sent Canberra's tree canopy and precious green space backwards. Labor's tired old agenda to maximise profits through land sales means our beloved bush capital is increasingly becoming Andrew Barr's multi-unit utopia," the spokeswoman said.

The amended motion which ultimately passed the Assembly was dramatically different to the one the Liberals originally put forward.

Mr Coe had sought to pin the blame on the Barr government for Canberra's high rental prices, which have reached a median of $575 a week.

His original motion said the government's land release, planning system and tax regime had stifled the supply of new houses for rent. It called on the government to ensure the pace of construction of new houses kept pace with demand.