Opposition Leader Jeremy Hanson has escalated his party's opposition to light rail, formally warning companies bidding to build the line that a future Liberal government will stop the project.
Mr Hanson and transport spokesman Alistair Coe wrote to the two international consortiums shortlisted by the ACT government for the public-private partnership last week, saying the Canberra Liberals had called for contracts to be delayed until after the October 2016 election.
Citing the planned $783 million cost, ongoing interest and operating payments, Mr Hanson said the opposition did not believe it was viable to proceed with the construction of the 12-kilometre line from the city to Gungahlin.
"We inform you that we have called on the ACT government not to sign contracts until the matter has been properly put to the people at the next election, and that a future Liberal ACT government elected to stop the project will honour the will of the people," Mr Hanson said.
The letter was sent to consortiums Activate and Canberra Metro, shortlisted by the government for the project in March. It comes amid continuing opposition to the project from the Canberra Liberals, who have repeatedly said they will do everything possible to stop light rail.
The warning comes after Mr Coe welcomed the Victorian government's decision to pay $339 million in compensation for cancelling the controversial East West Link tunnel contract in Melbourne.
The ACT opposition won't say exactly how much it is prepared to pay to exit contracts for light rail after the election, but in April Mr Coe described the Victorian government's pay out as "reasonable".
He suggested the figure meant ACT taxpayers could pay less than $30 million in compensation if light rail was cancelled by a Liberal government.
The position left the Liberals at odds with Prime Minister Tony Abbott, who backed the former Victorian Coalition government's $5.3 billion plans for the tunnel and said Labor Premier Daniel Andrews had set "a dangerous precedent" for future investment.
At the time, Capital Metro Minister Simon Corbell said ending the light rail contracts would cost a Hanson government "hundreds of millions of dollars" and thousands of jobs.
Lobby groups and supporters of light rail have accused the Canberra Liberals of threatening sovereign risk over plans to end the contracts.
The project will be a key issue in the October 2016 election.
Last week Unions ACT confirmed it was polling voters' support for the tram, and Chief Minister Andrew Barr used the budget papers to signal plans for a $375 million capital contribution once operations began.
Contract negotiations are expected to continue until mid-2016, with construction to begin on the Northbourne Avenue, Federal Highway and Flemington Road route before the election.
Services are due to start in 2019.
Mr Coe said the letters showed the Liberals were being upfront with the shortlisted consortiums.
"That's why we have written to them, saying we will stop the project," he said.
"The consortiums are on notice ... Canberrans don't want this project to go ahead and the government and consortiums should hold off signing any contracts."
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