The National Capital Authority could force the ACT government to realign its light rail route to Woden, substantially blowing out both the cost of the project and travel time, if the agency decided the tram would spoil the Commonwealth Bridge vista.
The Australian Parliament will also have to rubber-stamp the route over Commonwealth Bridge, National Capital Authority chief executive Malcolm Snow revealed in parliamentary hearings on Thursday.
Already the NCA, which has planning jurisdiction in the Parliamentary Zone, has said trams must be wire-free from the time they leave the Northbourne Avenue terminus, until a point yet to be determined down Adelaide Avenue.
But when asked if the NCA had the power to force the ACT government to use Kings Avenue Bridge rather than Commonwealth Bridge to preserve the Parliament House vista, Mr Snow acknowledged they did.
"If we felt the impact aesthetically and environmentally and physically of putting light rail infrastructure down the Commonwealth Avenue carriageways was not desirable we could certainly say it cannot go down Commonwealth Avenue, you'll have to go down another route," Mr Snow said.
However Mr Snow indicated this was unlikely, given the significant cost and time blow-outs.
"As you would appreciate with light rail with this form of mass transit, it's all about trip times, it's all about the shortest route between two points," Mr Snow said.
"It's not like a Melbourne tram which wanders all over the place. If it's to get the patronage it needs to be competitive to the car so you're talking a trip time between Woden and Civic of less than 20 minutes.
"Pushing light rail down Parkes Way, which would be down Constitution Avenue to Russell then down across Kings [Avenue], around Capital Circle would certainly add significant trip time and significant cost I would imagine but to your point Chair, yes if the NCA came to the view this landscape and Commonwealth Avenue was far too sensitive to introduce something as sensitive as light rail well we could veto that route."
But a spokesman for transport minister Meegan Fitzharris said the government was working closely with the NCA to make sure that didn't happen.
"Many of the technical, route and other matters raised by Mr Snow are presently being considered by the ACT government as it prepares a business case for light rail stage two," he said.
One would detour into Barton away from Parliament House and could extend to the Canberra Hospital. Public consultation on the proposed routes closed on Sunday.
The ACT government earmarked $57 million in last week's ACT Budget for early design work and procurement for stage two. Federal funding is under negotiation as part of the Commonwealth's "city deals".
Mr Snow previously raised the possibility of a third bridge to carry the light rail tracks in the void between the two structures that make up Commonwealth Avenue bridge.