Premier Ben Carroll has just weeks to decide the future of the Suburban Rail Loop, with tunnel boring machines scheduled to start by September in what would be a watershed moment for the project.

Crews at Burwood are preparing for two tunnel boring machines, already in the ground and pointing towards Glen Waverley, to launch within four to six weeks, but are allowing for unexpected delays that could push that out to eight weeks.

The tunnelling milestone is a thorny problem for the Coalition, who have committed to pause and review the project if elected in November, but also provides a ticking clock for the new premier.

Carroll has not made any immediate commitments about the loop’s future under his leadership but he is an internal critic of the project and has flagged further announcements and consideration of the matter through a cabinet process.

On Wednesday, he left the door open to reviewing the project and was “very focused” on delivering the stalled Melbourne Airport Rail Link.

Asked about reports he would reconsider the SRL, a pet project of former Labor premiers Daniel Andrews and Jacinta Allan, Carroll said it was a matter for his new cabinet.

“I’m going to run a disciplined, proper cabinet process,” he said.

But he went on to say he was not going to be a premier who broke contracts when answering a question about whether Victoria should have proceeded with the Commonwealth Games, currently taking place in Glasgow, Scotland.

“I’m going to be a premier that doesn’t break contracts,” he said.

Daniel Andrews famously cancelled the Commonwealth Games in 2023 and also ripped up a contract to build the East West Link road tunnel when he was first elected in 2014.

The state government has already signed contracts worth more than $14 billion to build the rail tunnel through Melbourne’s eastern suburbs.

Carroll affirmed his support for the Melbourne Airport Rail Link, which would include a new station in his electorate and had stalled under Allan’s leadership.

“Everyone knows there is not a stronger supporter for Airport Rail … than me,” he said.

“Airport Rail will electrify the north-west – it is a game changer, it’s been talked about for a hundred years.

“And as premier, I’m very focused on Airport Rail.”

The Age has spoken to three construction industry sources who confirmed tunnel boring machines for the Suburban Rail Loop were sitting underground at Burwood and on track to begin work by August or September.

They will tunnel towards Glen Waverley, with another two machines at the Burwood site to launch towards Box Hill next year. One of the machines is named Alice, after Melbourne musician Alice Ivy.

The Box Hill to Glen Waverley tunnelling is being delivered by Terra Verde, a consortium of WeBuild, GS Engineering and Construction Australia and Bouygues Construction Australia.

Once the tunnel boring machines begin digging, it is almost impossible to extract them from the ground and any cancellation of the project would likely involve them being buried.

The massive machines can, however, be paused for periods.

The government is also on the cusp of signing another multibillion-dollar contract for the loop – to deliver half the underground rail line’s six stations.

Place, a consortium of John Holland, Kellogg Brown & Root and Arup, has been shortlisted to build the stations at Glen Waverley, Burwood and Box Hill.

Two other bidders on the shortlist – ESPA and Connexus – are in the race for Cheltenham, Clayton and Monash stations.

Before Allan’s resignation on Tuesday, a government spokesperson told The Age that cancelling the project would mean the machines remaining buried.

“The only plan Jess Wilson’s Liberals have is to cut the Suburban Rail Loop – sacking thousands of workers, blocking more homes being built and leaving tunnel boring machines to sink into the ground,” they said.

Opposition transport infrastructure spokesman Evan Mulholland said the Coalition would “pause and review the Cheltenham to Box Hill line as the only financially responsible pathway going forward”.

“Ben Carroll never had the ticker to stand up to Jacinta Allan on the Suburban Rail Loop; in fact he publicly supported it because he has been too weak,” he said.

“Ben Carroll is tied to this boondoggle of a project through his actions of failing to stand up for Victorians as Jacinta Allan’s deputy.”

The four to eight-week deadline will be top of mind for Carroll as he considers whether to alter the project.

The Age has previously reported that a policy reset, which could include an adjustment to the loop, was flagged in early discussions about a potential spill in June, but that no concessions were ultimately agreed upon.

This year, tunnel boring machines are also expected to launch from Clarinda towards Glen Waverley, part of work being delivered by a separate consortium named Suburban Connect.

Further digging is scheduled for 2028, including a second Burwood launch towards Box Hill and a dig from Heatherton to Cheltenham.

The Suburban Rail Loop Authority did not respond to questions about the timeline of the first launch, but confirmed that trucks ferrying soil from the site would be able to directly access the Burwood Highway.

Over the weekend, an investigation by The Age showed how the Suburban Rail Loop is shifting away from its city-shaping purpose, with insiders dismayed that the project’s ambitious urban development plans have been scaled back.

The Age has also reported on a multibillion-dollar works package focused on the connection points between the rail loop’s new stations and existing Metro stations.

More on Victoria’s leadership spill

• Inside story: After an icy meeting, she still had hope. Then came a second knock at the door
• Leaked secret report: Allan was warned years ago about Big Build corruption
• Who is Ben Carroll? New premier’s journey from Kmart to power
• Comment: Who on earth can you vote for in this state?
• Slow, then fast, the downfall of a premier under siege