An Infrastructure Australia report on Melbourne's Suburban Rail Loop (SRL) urged the state government create an "exit strategy" in case the orbital train line cannot be delivered.
It called on the Victoria 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.
"Based on the information provided, we have low confidence in the cost estimate for SRL East, presenting a major risk to the SRL East project and the SRL Program as a whole," the report says.
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's due to be completed in four stages over several decades.
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.
The report found the economic benefits of the first two sections had been "overstated" and calls for more rigorous assessment to justify why the rail line is a priority for the state.
The independent statutory body also urged the federal government to hold off committing more money until Victoria hands over further analysis, updated costings and financial documents including its funding strategy.
Suburban Rail Loop Minister Harriet Shing said the state government was working with Infrastructure Australia but did not expressly commit to providing the requested information.
She said the project was within the expected price band and the government stood by the loop after taking it to two state elections.
"To be really clear, the Infrastructure Australia report is the basis upon which the recommendation occurred to release $2.2 billion (from the federal government)," Ms Shing told reporters.
"As we deliver long-term projects an orbital rail loop around the city, we will see benefit, not just for the people who immediately access the benefits of living and working closer to rail."
Opposition major projects spokesman Evan Mulholland described it as a "damning assessment" and urged the government to halt construction and review contracts immediately.
"This is sobering reading, it's a blistering assessment of (Premier) Jacinta Allan's vanity project," he told reporters.
"It obliterates the SRL business and investment case that was cooked up back in 2020."
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