Stage 2B of Canberra's light rail is 'proving technically difficult', National Capital Authority's Sally Barnes says

The National Capital Authority, which manages Commonwealth-controlled land, said the ACT government's "difficult" route proposal is under ongoing discussion.

Officials also revealed there could be further disruptions to pedestrians and cyclists along Commonwealth Bridge as works begin to construct a new bridge for the light rail to cross Lake Burley Griffin.

Authority chief executive Sally Barnes on Thursday said the preferred route for stage 2B of Canberra's light rail might have to be altered due to difficulties.

"We've had initial indications that may be proving technically difficult," Ms Barnes said, answering questions from Canberra MP Alicia Payne.

"It's not off the cards yet, but we're expecting to have discussions with the ACT government fairly soon about - would there be another way to traverse the national triangle?

"We're more than open to have a look at those other ways because if it's difficult, it's difficult."

The ACT government in 2018 said its preferred route for light rail's second stage would turn left into the Parliamentary Triangle at King George Terracce from Commonwealth Avenue. The indicative route would travel through Barton back to Adelaide Avenue.

However, then-ACT transport minister Meegan Fitzharris in 2019 said it was likely light rail would travel wound the eastern side of State Circle, dropping the so-called Barton "dog leg".

The latest precinct map includes stops at Kings Avenue, Sydney Avenue and Melbourne Avenue.

Light rail's role in providing public transport access for public servants accessing offices in and near the Parliamentary Triangle are set to form a key part of the territory government's pitch for additional Commonwealth funding for the project.

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Ms Barnes on Thursday did not reveal further details about what was causing the complications but added "improving public transport to the national cultural institutions is very important".

Construction of stage 2A of the project, along with raising London Circuit, has begun and is expected to take around four years.

This initial part of stage two will expand the light rail network from its current Alinga Street stop to Commonwealth Park.

Stage 2B will then extend the network from Commonwealth Park to Woden but final approval is not expected for many years.

The ACT government has not released a cost estimate nor completion date for the light rail link to Woden despite political and community pressure.

A new bridge between Commonwealth Bridge's north and south lanes will be constructed to link the network to Canberra's southern suburbs across the lake.

Chief planner Andrew Smith said complications had already arisen early on in the piece as a result of high voltage cables running along the bridge.

Instead, the cable relocation will happen in conjunction with works to strengthen Commonwealth Bridge.

Mr Smith said the changes will mean one of the bridges may be closed off to pedestrians and cyclists, forcing all foot traffic to the other bridge.

"It is inevitable that just by virtue of having to ... widen it to produce the active transit routes, that for a period, pedestrians and cyclists won't be able to use one of the bridges and so that's all north-south pedestrian and cycle traffic moving what's in potentially a very narrow space," he said.

"Within our budget, we've got quite a healthy allowance for safety measures to make sure that can occur."

The agency is also facing further complications with the type of grass it chooses to lay on the light rail's track.

Mr Smith said the authority had been pushing for grass over concrete and a prototype test track at the National Arboretum was proving the decision was trickier than expected.

"The vista, of course, down if you come from City Hill through to Parliament House is a very important part of the landscape setting of the city," he said.

"We have for longtime advocated, and the ACT government's agreed, [for] a grass track.

"But with that brings a whole lot of horticultural complexities."

- with Jasper Lindell