The first stages of Victoria's mammoth Suburban Rail Loop will not provide adequate value for money, according to independent analysis.
The Parliamentary Budget Office's latest assessment on Thursday revealed every $1 spent on the first two phases would have social benefits of between just 60 and 70 cents.
Opposition leader John Pesutto requested the audit in July 2023 to assess the decades-long project's cost-benefit.
The benefit-cost ratio (BCR), a key measure of a project's value, came in at 0.6 to 0.7 under normal conditions.
What this translates to is that for every $1 spent, Victorians would receive 60 to 70 cents in benefits, according to the analysis.
The net present value (NPV), which measures the project's overall value, was marked between -$7.4 billion and -$10.6 billion, meaning the project will deliver a net loss.
The two main sections, dubbed SRL East and SRL North, will form a 60km metro line through Melbourne's middle suburbs across 13 stations.
SRL East is currently under construction and is planned to open in 2035.
Construction costs for these sections of the tunnel would be about $96 billion, according to the report.
The cost to operate the link until at least 2084 would be another $120 billion.
However, the report said the project could have a net positive value up to $17.3 billion if it included "all expected benefits".
These included urban consolidation, and the wider impact of transport user benefits, infrastructure cost savings and improved social inclusion and access to employment.
Minister for Transport Infrastructure Danny Pearson said Melbourne's expected population spike over the next several decades is reason enough for the loop.
"This project just stacks up...we're going to create 70,000 homes along the corridor and we're going to provide modern transport infrastructure to Australia's largest university," he told reporters.
"We're going to be the size of London by the 2050s and that's going to come in the blink of an eye, that's why we have to get on and deliver this project."
Mr Pearson also took aim at the validity of the report's analysis.
"With major infrastructure projects, you never factor in operating costs...it is a dodgy question asked of the PBO," he said.
Deputy opposition leader David Southwick said the analysis was proof the project was too costly to justify.
"A benefit-cost ratio below one demonstrates the SRL will cost more money than it generates," he said.
"To saddle future-generations with hundreds of billions of dollars of debt to pay for it is financially reckless."
Mr Pearson alongside his federal counterpart Catherine King on Thursday announced $63.5 million to rebuild Sunshine Station, a site central to Melbourne's eventual airport rail link.
It was also announced that three train stations in central Brunswick will be closed and replaced with two new stations as part of a sky rail upgrades to the Upfield line.
