However, the potential fix is complicated by the proximity of a giant hole for Parramatta’s underground station, and ongoing negotiations between Sydney Metro and contractors.

The project is at risk of further cost increases even if the tunnel is dug deeper – there are claims running into the hundreds of millions of dollars by Malaysian contractor Gamuda for delays to work as a result of the problems, sources say. Halting the boring machines has required a resequencing of work at the Parramatta station.

Gamuda, which declined to comment, is leading a consortium that is building the nine-kilometre stretch of twin tunnels between Westmead and Sydney Olympic Park for the Metro West project. It is one of three major tunnelling contracts for the underground line, which will span 24 kilometres from Westmead to Sydney’s CBD once completed in 2032.

The estimated cost of tunnelling works at the line’s western end has already surged by $353 million from its original estimate to $2.7 billion, tender documents have shown.

Sydney Metro said it was aware of the matters relating to the Telstra building before awarding the western tunnelling package contract in 2022, and the contractors knew of them before starting work.

The agency said it was working with Gamuda and consortium partner Laing O’Rourke to finalise the design for the remainder of tunnelling and expected the boring machines to start again “very shortly”.

“Ongoing discussions between Sydney Metro and its contractors are commercial-in-confidence,” it said in a statement.

“Sydney Metro has been working with Telstra to co-ordinate geotechnical investigations at this site to verify ground conditions, foundations and pile depth of the building.”

The agency said geotechnical results had been used as input into the final metro tunnel design and alignment to “mitigate any risks to the buildings above”.

“Due to the complex function of the 213 Church  Street building, Sydney Metro has also completed investigations from an adjoining site to further verify foundation depths,” it said.

Telstra said it was working closely with the government and Sydney Metro to ensure its infrastructure was not affected by the new metro station.

Transport Minister John Graham said it was appropriate that the tunnelling machines “proceed with an abundance of caution” as they neared the site. “Thankfully, we expect the [tunnel boring machines] to be back in action very soon,” he said.

Shadow transport minister Natalie Ward said Sydney could not afford for Metro West to lose momentum and be delayed a second time.

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“However, taxpayers rightly deserve to know which expert signed off on the advice that has led to this issue,” she said.

Delays will create headaches for the government, which suffered a blow last month when Australian Turf Club members voted against selling Rosehill Gardens racecourse for $5 billion. The government had hoped to develop it into a new “mini-city” of 25,000 homes and build a metro station there.

Sydney Metro executives told a hearing in March that analysis provided to the government showed Metro West risks costing more than its $25.3 billion budget.