If the ACT government eventually abandons plans for a light rail line from the city to Gungahlin, taxpayers will still foot a bill of about $60 million, opposition transport spokesman Alistair Coe said on Tuesday.
Liberal members voted against the $23.5 million in funding in this year's budget to get the project to an ''investment-ready'' stage, as Mr Coe warned the 12-kilometre tram line, the largest-ever capital works project in the territory, was at risk of cost blowouts and delays.
Approved by Labor and Greens minister Shane Rattenbury, the funding for the Capital Metro agency includes planning and tender preparations and agency costs.
A further $20 million will be spent to improve the broader Gungahlin-to-the-city corridor, including widening of roads, intersection upgrades, moving of utility poles and wires, drainage works and pedestrian-infrastructure improvements.
Opposition calculations also included $10.7 million in spending since the 2011-12 financial year, plus $1.4 million for the government's light rail masterplan, $1.2 million from the Treasurer's advance and $96,000 in further capital funding.
Mr Coe claimed the government was pursuing the $614 million project because of an ''emotional attachment'' held by Capital Metro Minister Simon Corbell.
He continued to question projected patronage figures put forward by advocates of the tram line and said the government's management of the Cotter Dam construction, Gungahlin Drive extension and other projects meant light rail would likely be delivered late and over budget.
The start of construction on the Gungahlin line by 2016 was a key commitment of the parliamentary agreement with Mr Rattenbury which returned Labor to government after the last election.
Mr Corbell told the Assembly the construction of light rail would ease population pressures and reduce traffic congestion, including on Northbourne Avenue.
He derided the opposition for believing public transport infrastructure only benefited passengers.
''We still don't know whether the cost of relocating the pipes and wires that lie under the proposed route is factored into the government's $614 million of cost projection,'' Mr Coe said.
''The uncertainty of these costs has led the Centre for International Economics to say that light rail is a source of risk for the financial position of the territory.''
In its response to a report by the budget estimates committee, the government again indicated it might still walk away from the project after it receives the final business case in October – an increasingly common qualification in its public comments on the project in recent weeks.
Chief Minister Katy Gallagher has made it clear she will not support a project that costs substantially more than $614 million, adjusted from 2011 dollars.
The committee called for the final cost-benefit analysis to be made public before any contracts were signed, but the government said it would not release commercially sensitive information until contracts had been finalised.
''Certain details of agreed arrangements will be able to be made publicly available after an agreement has been reached between the government and a light rail delivery partner,'' it said.
The Assembly is expected to pass the 2014-15 budget on Thursday.