The federal parliament's joint standing committee has interrogated ACT government officials for the past few months over the decision to use Commonwealth Bridge to get light rail to Woden, amid concerns it will negatively impact on the Parliamentary Triangle.

Committee chair and Liberal MP Ben Morton has returned repeatedly to the question of whether Kings Avenue bridge would be preferable, given it would lend itself to an offshoot to the Russell offices and the airport.

That would require light rail to travel down Constitution Avenue, which is already earmarked for use on the future east-west spine between Belconnen and the airport.

However ACT Transport Minister Meegan Fitzharris said the government was unlikely to invest in the project if the light rail was forced to cross via Kings Avenue.

“Canberra is best served by a north-south light rail spine via Commonwealth Avenue, which will provide better light rail coverage through the City and Parliamentary areas, and result in higher passenger numbers, fewer significant trees needing removal and better travel times," Ms Fitzharris said.

“Up until now all our planning and design has been for a City to Woden route that crosses the lake at Commonwealth Avenue. Crossing the lake at Kings Avenue to get to Woden would increase the cost to around $1.9 billion, while isolating vast areas of the city.

"Given the lasting, sub-optimal outcomes this would produce for Canberra, the ACT government is reluctant to support it."

In responses to questions on notice, the government estimated the cost of building light rail via Kings Avenue would be between $1.53-$1.9 billion. By comparison, the Commonwealth Avenue route is expected to cost between $1.3-$1.6 billion.

These costings are not detailed and are based on a price per kilometre, given the Kings Avenue option is two kilometres longer than the Commonwealth Avenue route.

It also doesn't take into account the higher annual maintenance cost, or the need for extra light rail vehicles, drivers and staff, which could add up to an extra $7 million per year.

The government said using Constitution Avenue and Kings Avenue for both north-south and east-west services would reduce the overall capacity of the future light rail network, given the two lines will overlap on Constitution Avenue.

It would also limit future expansion of the network and would cause major traffic problems on Coranderrk Street.

The government analysed the intersection of Constitution Avenue and Coranderrk Street when considering the Russell extension during procurement for light rail stage one.

While the intersection was expected to cope with light rail services to Russell and the airport, it was expected to fail if it was also used for north-south light rail.

"In turn this would expose the city road network to unacceptable delays," the government said.

Already, Constitution Avenue is not wide enough for a separate light rail corridor, which exposes the network to delays from broken down cars and traffic.

According to its modelling, using Kings Avenue would blow out the travel time from Civic to Woden to 35-39 minutes.

Southside residents already say the 25-30 minutes the chosen stage two route will take is too long, and have called for the government to dump the Barton dogleg.

The government also said the Kings Avenue route would pick up fewer passengers, with a projected daily patronage of 31,200 in 2041, compared to 39, 000 for Commonwealth Avenue.

There are buses that replicated the chosen route for light rail stage two and the alternative Kings Avenue route -a standard bus service from Civic to Woden that down Commonwealth Avenue and through Barton; and a Rapid bus that goes via Constitution Avenue, along Kings Avenue and through Barton.

An analysis of MyWay data showed there were four times as many commuters travelling over Commonwealth Avenue rather than Kings Avenue.

The government also pointed out more Charles Weston trees would have to be chopped down if the light rail went down Kings Avenue - 40 compared to 28.

The ACT Labor conference recently voted for stage three of light rail to go from Belconnen to the Airport, and Ms Fitzharris said she would "be looking into how we can make that happen".

“Ultimately we are building a light rail network that needs dedicated corridors travelling east-west and north-south, both intersecting in the city," Ms Fitzharris said.

“We can’t look at each route in isolation – we need to think about how it will work within the broader network if we want to get this right.

“An alternative via Kings Avenue would ultimately be detrimental to both the road network and the light rail network as it would force both the route to Woden and a future route to the Airport to share the same track along Constitution Avenue, creating significant delays to both routes.”