Mr Barr said contracts wouldn't be signed until the proposed Alinga Street to Commonwealth Park extension had been approved.

The assessment process won't be finalised before the government enters caretaker mode on September 11, during which time it cannot sign major contracts.

Mr Barr expressed frustration with the slow-moving approvals process, saying the federal agencies responsible for assessing the project were understaffed.

The decision means light rail could once again become a key election issue in the ACT, with the Canberra Liberals still yet to declare their position on extending the line across the lake and on to Woden.

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Opposition transport spokeswoman Candice Burch has previously said the Liberals wouldn't tear up contracts signed before the election.

But confirmation that the government won't enter into a deal before October 17 means the Liberals have a choice to make.

Ms Burch did not respond directly when asked if the Liberals would push ahead with extending light rail if they won the election.

In a statement, a Canberra Liberals spokeswoman said it would order an independent assessment of the business case to ensure the best outcome for Canberrans.

The spokeswoman said the Liberals would announce their transport policy in the coming weeks.

Mr Barr on Friday confirmed an un-redacted business case wouldn't be released prior to the election, arguing its disclosure could undermine negotiations with potential bidders.

In an earlier statement, Ms Burch said: "Too many questions about this project remain unanswered."

"Perhaps the reason the Labor government hasn't come clean about the details of this significant infrastructure project is because they actually have no idea and are making it up as they go along," Ms Burch said.

Mr Barr joined Transport Minister Chris Steel on Friday to announce the release of a new report prepared as part of the approval process for the so-called Stage 2A of the light rail expansion.

The Commonwealth environment department ruled earlier this year that the 1.7-kilometre extension would not require a full environmental impact assessment.

But the department did request preliminary documentation, which needed to detail, among other points, how the ACT government intended to manage the potential destruction of populations of the critically-endangered golden sun moth.

This included moth habitat identified at the site of a planned "dedication traction substation" at Regatta Place, adjacent to Commonwealth Park.

The substation was proposed to help power the wire-free vehicles set to run on the new line.

But according to the government's report, the proposal has been scrapped after it was determined that a substation wasn't needed for the vehicles to be able to operate without overhead wires.

"Charging of the light rail will instead occur by means of the existing City to Gungahlin route with regenerative braking boosting the on-board power supply sufficient to support operation within the proposed light rail route," the report said.

However, large swathes of golden sun moth habitat would still be destroyed if the project gets the green light, according to the documents. More than three hectares would be destroyed through land clearing, while another two hectares would be lost via fragmentation.

The ACT government will purchase environmental credits as part of efforts to offset the damage to the critically-endangered species.

The report described the broad social and economic benefits of the project, which it argued was necessary if Canberra wanted to retain its status as one of Australia's most liveable cities. The project was expected to create about 300 jobs, mostly in construction.

"The project would help with stimulating the economy, through revitalisation opportunities. It would also help support future development and activation along the route, while reducing reliance on Commonwealth government employment and expenditure," the report said.

Public consultation on the report is open until September 14 - three days after the ACT government enters caretaker mode.

The report does not include the expected cost of the project, which the ACT government has also yet to disclose.

"We are committed to building a light rail network that benefits Canberrans," Mr Steel said.

"The growth in public transport patronage and the benefits that light rail has delivered along stage one, are the same benefits that we want to bring to Woden and the southside.

"With the planning approvals set in motion for the extension of light rail to Commonwealth Park, work will continue to refine the project's planning and design development with a view to construction starting as early as next year."