Early construction works on Melbourne's Metro Tunnel cost $150 million more than first planned, putting pressure on the rail project's $11 billion budget.
The initial construction-focused works were expected to cost $476.6 million, but that forecast has been revised to $625.5 million, according to Auditor-General Andrew Greaves.
However, money was saved in land acquisitions, reducing the total overspend to $95.8 million.
"The early works phase has cost more than the original total budget of $1.257 billion," Mr Greaves wrote.
"The final forecast for all activities related to early works is now $1.353 billion, an increase of $95.8 million, or 7.6 per cent."
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The increase is due to the added scope of the project, a shorter time frame, and issues at the State Library Station site.
The report raised concerns about the capacity to handle further blowouts in costing.
"(Rail Projects Victoria) had substantially consumed the project's overall cost contingency at the time of this audit," he wrote.
"This raises an early concern about the sufficiency of contingency funds available for the next phase of the project, which includes five years of complex and risky tunnelling and construction."
One of his 11 recommendations was to review the remaining contingency funds, taking into consideration the construction risks to date and likely future time and complexity pressures.
Transport Infrastructure Minister Jacinta Allan said the project will still be delivered within the $11 billion budget by 2025, and some of the added cost had helped mitigate potential problems further down the track.
"The Metro Tunnel Project is ahead of time and within project budget," she said.
Mr Greaves' recommendation had been accepted, she added.
Opposition transport infrastructure spokesman David Davis said the early works had been "botched" with errors in planning.
"Right across the whole project the early works (are) massively over budget, Labor can't manage these projects," he told reporters.
