Sydney's Metro West rail project makes secret tunnelling progress

Another mega tunnel boring machine has been quietly launched to dig out Sydney's latest Metro rail tunnel, despite a question mark lingering over the project's future.

The Metro West line from the CBD to Westmead, initiated by the former Coalition government, is now under a "value for money" review, with Premier Chris Minns repeatedly refusing to rule out cancelling it altogether.

On Wednesday, the third of six tunnel boring machines (TBM) began carving out the section of tunnel between Sydney Olympic Park and Westmead.

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A milestone, usually marked with much government fanfare, but there was no acknowledgement from embattled Transport Minister Jo Haylen.

Instead, the progress was noted only on the agency's website.

Speaking on 2GB Radio in late July, Minns said the project had "barely begun", but Sydney Metro's latest update said "approximately 138,000 tonnes of material has been removed" to carve out a 130-metre long, 20-metre wide and 28-metre-deep box at Clyde in preparation for Wednesday's launch.

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The premier continues to cite a major budget overrun, which has seen cost estimates for the project blow out to $25 billion, as a reason for the independent assessment.

Shadow Transport Minister Natalie Ward accused the government of "creating a problem that doesn't exist".

"The government's secrecy on Metro West is becoming farcical," she said.

"The government needs to honour its promise and commit to building Metro West."

The latest machine has been given the name "Betty" after much-loved Australian Olympian Betty Cuthbert, who was a Parramatta local.

In an Australian first, Betty is an autonomous TBM, allowing her to complete the tunnelling faster and more accurately.

She joins TBMs Daphne and Beatrice which were launched earlier this year from what will become the Bays Precinct station at White Bay.

TBMs Daphne and Beatrice have already tunnelled almost two kilometres from White Bay and are fast approaching Iron Cove where they will dig under Iron Cove towards Five Dock.

You can follow the TBMs' progress here.

Metro says another TBM will be launched at Clyde in the coming months to excavate the second metro tunnel alongside Betty.

The Sydney Metro Review won't be handed down until after the state budget.

The transport minister has been contacted for comment.

A 9News request for footage of the historic launch was denied.