Speaking at a Conservation Council forum on Tuesday night, opposition transport spokeswoman Candice Burch said the Liberals were now committed to extending light rail.

But she said the party would conduct an independent analysis to decide what the next stage of light rail should be, if it were to win the October election.

"The government is still not being transparent with Canberrans about what analysis they have done, about the business case and about the cost that is going to be involved with future stages," she said.

"Anecdotally a lot of people say to us 'what about Belconnen through the city and out to the airport, that seems to make the most sense as the next route'.

"We still haven't heard from the government as to why that's not the case.

"We want to be certain that the route and the money that we're committing to is the right investment commitment for Canberra and it's not simply about chasing votes."

Transport Minister Chris Steel said the south of Canberra needed to reap the same benefits of light rail as the north had.

"We are committed to expanding it beyond [Woden] with pre-feasibility work underway on stages three and four," he said.

The government has released a redacted business case as well as updated costings in last month's budget update.

It showed the cost of the city to Woden line was set to blow out to almost $2 billion.

"There is no excuse for the Canberra Liberals now, we've just heard they are not committed to the extension to Woden, they want to look at Belconnen," Mr Steel told the forum.

"We need to deliver this to all parts of our city and that means a north-south spine to Woden."

He said the Liberals' stance would undo years of planning and likely mean any new construction would be put off for decades.

"They have never been committed to light rail," he said.

"The Opposition Leader was the architect of their plan to rip up the contracts for the first stage at the last election. This newest thought bubble is a plan to abandon residents in Canberra's south."

Master Builders ACT CEO Michael Hopkins said the Liberals' announcement would send "shockwaves" through the construction industry.

"Our industry needs certainty of the work pipeline ahead," he said.

"Based on the policies announced so far in the 2020 campaign, the gravity of the economic challenge ahead for the ACT is not yet understood by candidates."

Public Transport Association of Canberra chair Ryan Hemsley said the Liberals needed to put money on the table to show they were committed to light rail.

He said stage 2A of light rail - from the city to Commonwealth Park - should proceed because planning, approvals and design was at an advanced stage.

But Belconnen could make sense as the next stage, allowing the complex approvals and planning for 2B - Commonwealth Park to Woden - to occur in the meantime.

"I have my suspicions that they may not invest in any future stages of light rail," Mr Hemsley said.

"It all comes down to what money is allocated where.

"We need a little bit more than virtue signalling - they've had four years to come up with a proper policy.

"I fully expected they were banking on stage one to be a massive flop."

Stage one of Canberra's light rail network was completed in 2019, running from Gungahlin to the city.

Stage two is slated to run from the city to Woden.

The government went to the last election promising to have the contracts signed for the second stage of light rail before the 2020 election.

However the contracts are yet to be signed, with Mr Steel blaming the delays on the coronavirus crisis.

Under the government's current plans, stage three would create an east-west route from Belconnen to the airport.

Stage four would extend the Woden line to Tuggeranong.