Commonwealth Avenue Bridge could be replaced with an entirely new structure under a proposal to be investigated as part of planning for stage two of light rail.
The National Capital Authority and ACT government have agreed to start investigating the feasibility of building a new bridge over Lake Burley Griffin.
It comes as Chief Minister Andrew Barr this week said the expansion of light rail to Woden probably won't be completed until 2025, after the Coalition's surprise election victory left the ACT needing to find an extra $200 million to fund the project.
The authority's chief executive, Sally Barnes, told a Property Council breakfast on Tuesday it would be "silly not to look" at the option of a new bridge, given both parties were already planning for separate, significant upgrades to the landmark structure.
The authority has identified work is needed to strengthen and widen the bridge, replace safety barriers and improve access for cyclist and pedestrians.
The ACT government has been planning to build a separate Commonwealth Avenue bridge between the existing structures. It would carry light rail over Lake Burley Griffin towards Woden. The government had originally intended to run light rail over an existing traffic lane, but tweaked the plan after the authority raised concerns about traffic.
Ms Barnes told The Canberra Times the two parties would hold workshops from next month to explore the feasibility of a new bridge. She hoped to have "pretty clear answers" on whether it was a legitimate, cost-effective option as soon as August.
"It could be that you start to do that work and it [a new bridge] is either too expensive, or you think 'No, I don't want to get rid of those two bridges because they are quite magnificent'," she said.
"But it would also seem remiss not to at least ask questions."
Ms Barnes would not speculate on the possible cost of a building a new Commonwealth Avenue Bridge, but said repair work to the existing structure had been estimated at between $70 million to $100 million.
She said the authority would continue to prepare the business case for the structural upgrades, but said they would not go ahead if it was decided that a new bridge would be built.
The almost 60-year-old bridge had been well maintained, but had "reached the part of its life cycle where it needs a whole lot of work to keep it going for another 60 years".
If the Commonwealth Avenue Bridge was to be replaced, she said it would have to be with a "magnificent structure" which was well-designed and sympathetic to its surrounds.
On Monday, Mr Barr said the time frame for the delivery of stage two of light rail would be pushed back after federal Labor, which had pledged $200 million towards the project, was defeated in Saturday's election.
Labor's promised funding would not have come through until the 2021-22 financial year, but the territory was hopeful of presenting a proposal to federal parliament next year.
Ms Barnes said the authority's role in planning for the project would not be affected by the revised timeline.
A spokeswoman for Transport Canberra and City Services said the government was working with the authority to investigate plans for a new bridge over Lake Burley Griffin.
"At present, the ACT government is assuming a new light rail bridge will be built between the existing structures," the spokeswoman said.
"However, as the authority has received funding to assess works required to the existing bridges, and as light rail to Woden will need to cross the lake, it makes sense to work collaboratively to ensure the best outcome for all parties is achieved."