Transport Minister Meegan Fitzharris said Labor would commit to build the second stage before the October election, with contracts expected to be signed and work to begin before the 2020 election.

Labor would start construction on stage two as work on stage one finished, to keep the employment pipeline going and to move directly from one route to the next.

"We wouldn't be starting the second stage until the first stage is operating but we wouldn't want to have a large gap," she said.

The first stage, the 12-kilometre line from Gungahlin to the city, is to be finished in late 2018 or early 2019. Ms Fitzharris said stage 2 would probably be a similar length to stage 1 – about 12 kilometres.

The government says it has identified the four most popular routes from consultation with almost 900 people in late 2015. They are:

One of the big questions is how to get over the lake, with Planning Minister Mick Gentleman saying Commonwealth Avenue bridge had the advantage of three lanes in each direction, compared with two lanes for Kings Avenue.

Both options would require bridge strengthening work and both needed the involvement of the National Capital Authority, with the authority already indicating that trams in the Parliamentary Triangle should operate without overhead wires.

The bureaucracy had done "pre-feasibility studies" looking at engineering, patronage, increases in land value, and economic opportunities, Mr Gentleman said. "Once the next route was chosen, more formal studies would be done, including engineering studies on the bridges and work on the inclines in the Parliamentary Triangle.

"Whether we use the Kings Avenue route or the Commonwealth Avenue route to cross the lake will depend on the engineering studies and the feasibility studies," he said. "We know that Kings Avenue bridge is a thinner bridge than Commonwealth Avenue bridge so we will listen to the engineers."

The ACT would also want federal funding for a Parliamentary Triangle route.

Liberal transport spokesman Alistair Coe said government's "lines on a map without any analysis or costings" were "an insult to the intelligence of Canberrans".

"These proposals will cost billions of dollars, but just like the first stage, the government is more interested in its utopian ideals than dealing with the real costs that will be lumped on the Canberra community."

The government previously identified a 3-kilometre extension to Russell as its stage 2, but shelved the proposal in March, when Chief Minister Andrew Barr said he would begin work on a much more ambitious extension, taking in not only Russell, but the wider Parliamentary Triangle, and possibly also Canberra Airport and the Australian National University. The government asked the two consortiums bidding to build and operate stage 1 to submit bids also on the Russell extension, but has refused to release costings.

The government also surveyed almost 5500 people this year on public transport, with six focus groups, onboard and online surveys and phone interviews. It found only six per cent of people mainly used the bus, and only 20 per cent said they would consider using a bus. Ninety-five per cent of households have access to a vehicle.

The main reason given for people's most recent use of public transport was their car not being available – because it was being repaired or serviced or used by other family members.

The major barriers to public transport were that it took longer or that people wanted to run personal errands. Other significant barriers were people wanting a car for work, not liking having to change buses during a journey and buses not running often enough.

While 75 per cent of people had access to a bike, just 20 per cent said they would consider riding to work, and just five per cent actually did.