But the Barr government warns the price could fluctuate, depending on when the project gets under way.

The first details about the Civic to Woden line were released, as the ACT government prepares to give evidence before a federal parliamentary committee probing the impact of the proposal on the Parliamentary Zone on Thursday.

The government says if the Commonwealth approves the project this year, it could start looking for someone to build and operate it next year and construction could begin in 2020-21.

According to this timeline, the light rail could be up and running by 2023-24.

But the territory is yet to decide what its final investment will be, pending Commonwealth support for the project and any planning constraints.

The cost of stage two is said to be "commensurate" with the original cost estimate for the first stage of light rail, adjusted for inflation and taking in the extra costs of travelling across the lake, extra trams and the wire-free stretches.

However the final price will depend on the ultimate design of the project, when it is built and whether the national construction market is still overheated.

And while the government said the initial costings should be treated with caution, it said they should also be seen in context with its broader vision for public transport across Canberra.

"The ACT government will look beyond simple benefit cost ratio modelling in making its investment decision, and will take into account matters such as its overall vision for Canberra, community sentiment, urban realm benefits and other factors," their report says.

The government has ditched the idea of having light rail running on existing traffic lanes of Commonwealth Bridge and will instead build a dedicated bridge in the centre.

The Commonwealth Avenue crossing was chosen to alleviate traffic problems already occurring in the Parliamentary Zone and to serve the Australian National University and West Basin areas.

It was also chosen because a Kings Avenue alignment would have increased the Woden to city journey time by an extra five to seven minutes.

The trip from Woden to Civic has been modelled at 25-30 minutes already, and the extra time on the Kings Avenue alignment would have reduced patronage by one-fifth, a downturn which would not have been ameliorated by demand between Russell and the Parliamentary Zone.

The light rail will be wire-free from Alinga Street to Sydney Avenue, and again in Woden from the Phillip Oval stop. However overhead wires will be needed on Adelaide Avenue.

Initial engineering work showed long sections of the Adelaide Avenue alignment would be "constrained or impractical to function" without overhead wires.

However as this stretch includes the Lodge, more design work will be done to minimise the aesthetic impact.

Close to 30 trees would be pulled out of the Commonwealth median from Lake Burley Griffin to Coronation Drive.

However the trees are said to be in decline, and the plan is to replace them with 40 new trees and plant more in areas that have not had trees before.

In Woden, it is predicted hundreds of thousands of square metres of land could be opened up for development because of the light rail.

That includes 601,300 square metres of residential land - enough for 4070 homes - 679,000 square metres of commercial land, 49,200 square metres of retail land and 7500 metres of community land.

Parts of Callum Street in the Woden Town Centre would be closed to cars, with a shared zone built for the light rail, pedestrians and cyclists and the Woden bus interchange would be overhauled.

The area surrounding the national institutions will also be transformed with King George Terrace closed to private vehicles from the intersections of Parkes Place East and Parkes Place West.

This means the area in front of Old Parliament House will become a shared zone for pedestrians, cyclist and the light rail.

The government said it chose to exclude cars from its shared zones as a single road delay or breakdown would disrupt the light rail journey time.

About 130 car parks would be lost in the Parkes/Barton area because of the light rail.

However that is only 4 per cent of the car parks available in the area and the tram can move up to 4000 people into the region during the morning peak hour.

The goverment will require four levels of approval for light rail stage two to go ahead, including from the federal Environment department, the ACT Planning Directorate, the National Capital Authority and finally both houses of federal parliament.