The Victorian Government has had to justify the business case underpinning a multi-billion dollar tunnel project, which is already underway.

Premier Denis Napthine says the reported traffic predictions are solid, despite some media claiming they had been boosted for the sake of the report.

Fairfax Media has published what it claims are cabinet reports showing the numbers were tweaked. They allege that the numbers were boosted based on the assumption that rising wealth will encourage people to use toll roads. Supposedly, the thinking is that with more money to spare, people will be less likely to want to sit and wait in traffic.

Premier Denis Napthine says once the road is built, commuters will want to use it.

Napthine expects: “people will increasingly value their time, people will increasingly not want to be caught in congestion... so this project is about reducing congestion, improving efficiency in transport and that people will be prepared to pay for that,” he said.

The Victorian opposition says the business case was distorted to push the project ahead. Protestors have already made their opinions known, halting early drilling work at a number of sites.

Shadow Minister for Infrastructure, Brian Tee, has also questioned the figures in the business case.

“What they clearly show is that this modelling has never been used before, it's shonky... It achieves the outcome that the Premier wants rather than has an honest look at the merits of this project,” Mr Tee said