“If we’ve got tunnel boring machines which have gone six kilometres underground then we can hardly fill it back in,” he said.

“The project will then proceed, there’s no doubt about it – we’ll have to make sure it’s managed appropriately and unlike every other major project, it’s managed on time and under budget.

“No government would have any choice. If you’ve got tunnels that are half built, you can’t fill them in, we don’t intend to do that.”

Asked if the party would consider slowing down the project, Guy said: “If a project is commencing then a project goes ahead, it’s as simple as that. We’re asking this government ... this is not the project for Melbourne, please pause it when you’ve got the chance.”

Speaking later on Tuesday morning, Battin said he was in agreement with Guy’s position but denied there had been a shift in policy.

Battin confirmed that should the government not pull out of the project by December, when tunnel boring machines are due to arrive in the country, then the project will continue under the Liberals.

“We will review it and every step of the contract we’ll have a look at to see if we can save Victorians money,” he said

The $34.5 billion SRL East would connect Cheltenham and Box Hill through a rail link slated to be open by 2035. It would be the first stage of an orbital rail line that is slated to eventually connect all the way through to Werribee, via Melbourne airport – but later stages are not proposed to commence for decades.

Tunnel boring machines are due to arrive in Australia late this year, with tunnelling set to begin in 2026.

Suburban Rail Loop Minister Harriet Shing said she welcomed the “opposition’s confirmation” it would continue with the SRL in the event it wins next year’s election.

“I think that is something people should take a measure of comfort on, in consistency and on a project that the city needs and that people have actually endorsed four times in a row when they’ve headed to the polls,” she said at the ABC forum.

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The Liberal Party’s softening of its stance on the SRL sets the scene for the first election campaign since 2014 to not be dominated by discussion of whether the project will go ahead.

But Battin sought to cast doubt on whether the tunnelling would actually start before the election.

“Let’s wait and see what happens when it comes to boring machines and if the government can get anything on time,” he said.

“But if there are two holes in the ground and the boring machines have started, we will go through the process and I guarantee you we will manage that project better.”

An Infrastructure Australia report released in March said no federal money – beyond the $2 billion already contributed – should be committed to the loop’s first phase until it was clear the project stacked up financially.

The report urged Victoria to develop exit strategies to provide a clear pathway out of the project in case it couldn’t be delivered, raising concerns the state will have to fund $34.5 billion by itself.

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Victoria is planning for the federal government to cover a third of the SRL East, a third by itself and the final third through “value capture” – a mechanism to seek contributions from businesses and landowners who benefit from rising property values, though the feasibility of raising over $11 billion this way remains a subject of debate.

Public Transport Users Association spokesperson Daniel Bowen said the opposition’s commitment gave some certainty to the project, assuming that tunnelling begins.

“Whoever wins that election, they are going to need to take care to keep the costs under control, look for opportunities to improve the design and get on and deliver it,” he said.

“It has gone through two elections and it’s not unreasonable for Labor to push ahead with it, but they do need to make sure the budget is kept under control.

“And we won’t get the full benefits unless you get the full project so they do need to start thinking about how SRL North will be developed.”

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