It believes that having a diversion through Barton, rather than a more direct north-south route, would slow the network and so reduce the incentive to leave cars at home.

The institute's chief executive, David Williams, said politicians had to decide on their priorities for the system - moving lots of people, or making it speedy so distant commuters left their cars at home.

"It appears to be confused at the moment and we need it clarified by the politicians", he said.

The ACT government's current favourite proposal is to have the tracks running south over the lake along Commonwealth Avenue and then turning through Barton, rather than heading straight towards Woden.

The institute's planning officer, John Brockhoff, said, "The North-South transport line makes sense. Once it does the dog-leg, it slows down."

Far better than the dog-leg, he reckons, would be to get the north-south spine in place and then build on that with a spur to Barton and then, ideally, further out.

On top of the doubts of the Planning Institute of Australia, there are more fundamental questions from the Productivity Commission, which is part of the Treasury, and advises the Commonwealth government.

It thinks trams are much more costly than buses and so questions the need for a rail network at all.

Its submission to the ACT government inquiry says a rapid bus service would be a quarter of the cost of a tram service.

"Government resources are limited and there are many other calls on the public purse that are likely to be more highly valued than the ACT light rail project", Commissioner Paul Lindwall said in the submission.

Some of the Productivity Commission's scepticism relates to the mechanics of a fixed tram system versus a more flexible network of rapid buses - simple matters like if a tram breaks down, every tram behind it is held up but if a bus breaks down, everything behind it goes around.

Experts within the Productivity Commission also say there is evidence costs sky-rocket in this kind of major project.

On top of these doubts voiced by the Productivity Commission and the Australian Planning Institute, the peak body for planners on the proposal has urged the Barr government to rethink the route in the wake of a federal government review that found it would be easier to get the green light for a line that stuck to Canberra's major avenues.

The Joint Standing Committee on the National Capital and External Territories published its report on the extension plan last week, outlining a more complicated approvals process if the government continues to pursue its desired route.

A Canberra-wide plan released in 2015 had seen the Civic to Woden stretch of light rail go past Parliament House, although a government survey found nearly three-quarters of people preferred the Barton detour.

However the National Capital Authority told the committee the National Capital Plan - which allows for an intertown public transport system along Kings and Commonwealth Avenues and State Circle - could be considered as "in-principle" approval for light rail routes.

It warned any route that wasn't covered by this plan - like the Barton route - would "require information that is much more detailed than that which appears to be currently contemplated by the ACT government before a decision could be made".

The committee recommended a two-stage Commonwealth approval process if the ACT government continued to pursue the Barton route.

Transport Minister Meegan Fitzharris indicated on Monday the ACT government would still pursue the Barton route, and had always intended to file a National Capital Plan amendment to get it approved.

She told ABC radio she was wedded to the Commonwealth Avenue Bridge but undecided on the route in the Parliamentary Triangle. "We'll weigh that up", she said, "and make sure we deliver light rail to Woden."

However committee chair, Liberal MP Ben Morton told The Canberra Times there was no guarantee the amendment would be approved.

"It is not the committee’s intention to slow or hinder the approvals process, but rather to provide certainty for the ACT government and the people of Canberra, and to ensure that time and money is not wasted pursuing a route that is not endorsed by the Commonwealth and therefore not feasible," he said.