The party said a cost-benefit analysis of the Woden extension should include stops at Phillip and Mawson and the possible expansion of the Mawson park-and-ride facility.

But the duplication of Athllon Drive between Mawson and Woden would need to be stopped in the meantime to make sure a light-rail project did not need to tear up new road.

The party said the plan would prevent Mawson from being overlooked for light rail, which it said would be "shelved for several years" after attention shifted to the third stage to Belconnen.

"There is zoned land available for release in Mawson and Phillip that would be directly served by light rail to Mawson. Part of the cost of light rail to Mawson would be offset by higher land sales revenue," the party said.

Greens candidate for Murrumbidgee Emma Davidson said light rail to Mawson would be beneficial to southside residents in the same way it had benefited the north.

"Given that light rail from the city to Woden has been split into two stages, and that the second stage has been delayed by complicated federal planning approvals for the Parliamentary Triangle, there is now time to better consider the Mawson extension," Ms Davidson said.

"We can help keep costs down by getting this work underway at the same time as construction gets underway for the initial Civic to Commonwealth Park section.

"Considering this option in the business case will ensure the right cost-benefit assessments are completed before we commit to an extended route."

Stage 2A of light rail is planned to extend the route from its current terminus in the city centre to Commonwealth Park before stage 2B takes the route to Woden. The split project is to allow for the Commonwealth planning approval process.

The ACT government has refused to release full costings or cost-benefit analyses for the project, saying the documents are commercial-in-confidence.

The business case is not expected to be released before contracts are signed.

The Greens said the potential expansion would give passengers from the southern part of the Woden Valley and Tuggeranong better access to light rail, but buses from Tuggeranong would still terminate at Woden.

In a policy document to be published on Sunday, the Greens said the route from Woden to Mawson was short, flat and did not require a major bridge.

The party argued the cost of construction would likely be reduced because staff and equipment would already be in place after the route reached Woden.

Last month, it was revealed in the latest territory economic update the cost of the second stage of light rail could be almost $2 billion, blowing out by up to $300 million on earlier estimates.

"The total cost for completing light rail stage two, including design, construction, purchase of additional light rail vehicles, an additional depot, and project management is estimated to be in the range of $1.5 billion to $1.9 billion," the update said.

Liberal leader Alistair Coe was forced a fortnight ago to backtrack on a suggestion from Liberal candidate Candice Burch the party would do a study on where the second stage of light rail should go.

"The next stage of light rail will be from the city to Woden," Mr Coe said on September 17 after Ms Burch suggested the party would explore extending light rail to Belconnen first.

Mr Coe has committed to an independent review of the light rail business case, which Labor Transport Minister Chris Steel has called a signal of the Liberals' opposition to the project.

"The only policy the Canberra Liberals have when it comes to light rail is to obstruct, deny and delay," Mr Steel said, adding a review would add years in delays.