The ACT Greens put light rail for Canberra at the heart of their election pitch yesterday, launching a $200 million policy to start work on a system in the city within three years.

But the party stopped short of making the plan a non-negotiable condition of support for a major party if the Greens retain balance-of-power status in October's territory election.

Greens' transport spokeswoman Amanda Bresnan said her party would establish an ''independent statutory authority'' to oversee the planning and building of the system.

The party's four MLAs and Molonglo candidate Alan Kerlin made the announcement in Civic at the junction of Northbourne Avenue, Commonwealth Avenue and London Circuit, but Ms Bresnan said the first leg of the system would not necessarily be at the congested thoroughfare.

''We're committing $1.4 million for a Canberra-wide light rail master plan,'' Ms Bresnan said.

''So we're not looking at a particular part of Canberra, we're looking at the whole of Canberra.

''This is the first time that this commitment has been made and it's about setting up a process that removes the politics from light rail and ensures that light rail is actually implemented in Canberra.''

Ms Bresnan said she was prepared to submit the policy to ACT Treasury to be costed before the election.

''The $200 million is in capital funding and as with all the Greens' policies, it has been fully funded and they will be submitted to Treasury and what we're putting forward today is a realistic and responsible plan for light rail,'' she said.

But Ms Bresnan said the policy would be ''up for negotiation'' if her party found itself in talks about supporting either Labor or the Liberals after October's election.

''As with all the policies we are putting forward, everything is up for negotiation and discussing when you come forward, as it was at the last election,'' the Greens MLA said.

The policy won immediate praise from climate change pressure group Canberra Loves 40 per cent, the Canberra Business Council and public transport activists ACT Light Rail who called it ''the best policy announcement on public transport by any political party since self-government''.

But the government was quick to criticise the Greens' announcement yesterday with environment minister Simon Corbell accusing the cross-bench party of walking away from a solution to congestion in the Gungahlin-to-city corridor.

''Labor's position is that the Gungahlin-to-city corridor is the first stage and the sensible place to start for rapid transit in the city,'' he said.

''We're doing detailed investigations and costings and we will be releasing those detailed costings and our preferred vehicle for that corridor in the coming weeks.''

The Canberra Liberals, who have yet to announce a public transport policy, joined in the criticism of the Greens' plans yesterday with opposition transport spokesman Alistair Coe branding the policy as ''irresponsible''.