On Wednesday, it was revealed the trip on stage two of light rail to Civic to Woden via the Barr government's chosen route will take around 25 minutes.

It is five minutes longer than an alternate route proposed that went past Parliament House.

The current blue rapid bus that connects the two centres takes about 15 minutes.

Those living on the southside say their trip times into the city will suffer as a result of the change to light rail.

The Woden Valley Community Council said travel times would double for Woden residents, with even more blowouts for people living in Tuggeranong.

But ACT government officials appearing before a federal parliamentary committee examining the impact of the project on the Parliamentary Triangle on Thursday said the Barton route was preferable in a number of ways.

Transport Canberra deputy director-general Duncan Edghill said an overwhelming number of people supported the Barton route when surveyed last year, and it would take in more employment hubs, national institutions and hotels than the more direct route.

He said while the Parliament House route may be more direct, it would likely be more expensive because of the extra heritage constraints, the need to build another bridge and more pedestrian crossings.

However he was unable to provide any alternate costings that the $1.3-1.6 billion figure released on Wednesday.

He also chose his words carefully when asked if the light rail would take longer and run less frequently than the current bus.

"People's journeys will change," he said.

"Upwards?" committee chair Ben Morton asked.

Transport Canberra director general Emma Thomas stepped in, saying light rail and the blue rapid should not be compared directly.

"It's not just from Woden to Civic people are travelling, there's a whole range of journeys," she said.

Mr Edghill said it was yet to be decided if the blue rapid would be dumped once the Woden leg of light rail came online.

The government's submission to the inquiry said the idea of a Kings Avenue route had been quickly scuttled because it would have taken five to seven minutes more, reducing patronage by one-fifth.

A spokeswoman for stage two light rail said passenger modelling showed about the same level of people catching the light rail both on the Barton and Parliament House routes.

"Although the Parkes and Barton route has a longer travel time, this is offset by people wanting to travel to places of employment and interest in the area," she said.

Officials were also questioned on the choice of Commonwealth Avenue over Kings Avenue.

Mr Edghill said that alignment allowed them to pick up passengers from the Australian University.

Mr Morton asked whether Parkes Way had been considered as an alternate route to take the tram via Kings Avenue bridge to Woden.

"Parkes Way has other functions that don't lend itself to light rail," Mr Edghill said.

Mr Edghill also revealed the cost of building the bridge in between the existing Commonwealth Bridge for light rail would be roughly the same as building infrastructure on existing lanes.

"It's actually easier to build a bridge in between. There's complications with retrofitting an old bridge," Mr Edghill said.

In a bizarre twist, while the Commonwealth Bridge is the owned by the federal government, it appears the ACT government will own the bridge in the centre.

"We're paying for it," Mr Edghill said, by way of explanation.

Another bridge would need to be built if the light rail were to use Kings Avenue as well.

Mr Edghill admitted the idea of tunnelling under the lake had been briefly considered, but quickly scrapped due to cost.

Representatives from the National Capital Authority looked at each other when asked which bridge they would prefer the light rail to use.

"We'd prefer to see more traffic modelling first," chief executive Sally Barnes said.

The National Capital Authority has also indicated they would prefer Adelaide Avenue to be wire-free on the approach to Parliament House.

However Ms Thomas said current technology would not permit the light rail batteries to hold a charge for the 2.3 kilometres in between the last Barton stop and the Hopetoun Circuit stop, meaning wires were required.

Mr Edghill said wires allowed the light rail to reach its top speed, whereas running from a battery meant it had to go somewhat slower to conserve charge.

However he noted the light rail would be running in 2024 at the earliest, meaning the technology could have changed by then.