Victorian Opposition Leader Brad Battin wants Premier Jacinta Allan to immediately suspend all contracts linked to the Suburban Rail Loop.
The proposed 90km orbital rail loop is intended to run from Cheltenham in Melbourne's southeast to Werribee in the outer west via the airport at Tullamarine.
It is due to be completed in four stages over several decades.
The opposition previously wanted the project paused, but has hardened its stance.
"Cancel the contracts as soon as possible and save Victorians from going broke from your pet project," Mr Battin said on Monday.
The call follows fresh doubts raised by the national independent infrastructure assessment body on the rail loop's cost and timeline to completion.
Infrastructure Australia found the SRL's benefits had been overstated and urged the government to create an "exit strategy" in case the orbital train line cannot be delivered.
It also called on the government to release up-to-date costings as the $34.5 billion price tag for the first section of the line from Cheltenham to Box Hill, known as SRL East, was calculated in 2020 before construction costs soared.
Tunnelling contracts worth $5.3 billion were signed in 2024, ahead of boring machines being turned on in 2026.
The Victorian government is relying on the Commonwealth to fund one third of SRL East and so far it has handed over $2.2 billion.
Asked about further funding, Prime Minister Anthony Albanese said the federal budget on Tuesday would have "substantial" infrastructure investment and declared the SRL a viable long-term project.
Infrastructure Australia wants the federal government to hold off committing more money until Victoria hands over further analysis, updated costings and financial documents.
"This government is so incompetent they've signed contracts for a project that would effectively send this state broke without knowing where the funding is coming from," Mr Battin said.
The opposition leader did not commit to scrapping the project entirely if he wins government in 2026, instead vowing to pause and reassess it.
Transport Infrastructure Minister Gabrielle Williams on Monday said the report highlighted the need for the government to be more transparent about the project to build confidence.
The entire rail loop was initially estimated to cost $50 billion when first announced by Labor before the 2018 state election.
The 2021 business case showed the east and northeast sections could cost up to $50.5 billion.
Further doubt was cast over the build cost by the state's independent Parliamentary Budget Office, which put it at $96.4 billion for first two sections in an 2024 estimate update.
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