Rail loop's first stages could cost $125b

On top of the $125 billion, it would cost an estimated $75 billion to operate the first two stages of the project in its first 50 years, according to Victoria's Parliamentary Budget Office.

The office calculated the total $200 million estimate at the request of the Victorian opposition, with Shadow Treasurer David Davis on Thursday calling the findings an "alarming revelation".

However it was unsurprising given Labor's "record of cost blowouts", he said, and the estimates raised serious questions about the suburban rail loop's viability.

"When Victorians in need can't get an ambulance, an answer to a triple-zero call or access critical treatments, it's clear this is the wrong project at the wrong time," Mr Davis said.

Opposition Leader Matthew Guy has vowed to shelve the loop, pouring money for the eastern arm of the project into Victoria's health system instead.

The budget office's findings go significantly beyond the Victorian government's initial estimates, with it originally slated to cost $50 billion for the entire line from Cheltenham to Werribee.

The government has put the cost of the eastern stretch at between $30 billion and $34.5 billion, while the budget office anticipated it would come in around $33 billion. 

The parliamentary office pointed to significant economic benefits of the rail loop project, including a benefit to cost ratio of 1 to 1.7.

It also noted there was insufficient publicly available information to cost some elements of the eastern and northern stretches.

A Victorian Liberal Nationals government would "prioritise" the state's health system by temporarily shelving the infrastructure, as it wasn't possible to do both at the same time, Mr Guy said on Wednesday.

With early works already happening, though, the opposition wouldn't rip up signed contracts. 

In question time on Wednesday, Premier Daniel Andrews was among four Labor cabinet members to defend the project, saying it wasn't a choice between one or the other.

"You need to do more than one thing at a time. That, it would seem, is beyond the capability of some," he told parliament, noting the project would become more expensive if put off.