The Victorian government is standing by allegedly flawed modelling which it’s using to underpin the planned $5.5 billion West Gate Tunnel.
Consultant William McDougall was hired for the project but says he was removed after raising with Treasurer Tim Pallas concerns about projected traffic levels.
He's told a senate committee there was substantial "optimism bias" in the government's review of toll road company Transurban's proposal.
But Roads Minster Luke Donnellan has restated his confidence in the modelling, which indicates about 67,000 vehicles will use the tunnel each day.
"More than a dozen independent experts with decades of experience worked on the modelling that underpins this project - their work is world class and unequivocally supports building the West Gate Tunnel," the minister told reporters on Monday, before championing the financial return of the toll road.
"When you put (those figures) through the business case that's a return of 1.30, so for every dollar you spend it will return $1.30.
Mr Donnellan further denied he had misrepresented the traffic forecasts and said the tunnel project will slash congestion and reduce travel times.
Mr McDougall was critical of how the traffic projections had been manipulated.
"This is a polite term for what I consider to be deliberate distortion and misrepresentation of traffic forecasts and the economic benefits that flow from them," he said.
Labor took power in 2014 and immediately axed the previous government's proposed $1 billion East West Link because the business case "didn't stack up".
Mr McDougall said he was "surprised" to see the same experts who did the East West Link business case then working on the West Gate Tunnel.
"After much deliberation, in the face of frustration and concern that the work would not stand up to scrutiny, I raised concerns at a higher level," he said.
"Shortly after doing this, I was taken off the (West Gate Tunnel) review work without explanation."
Opposition roads spokesman Ryan Smith labelled the tunnel as a "dud" and said the government "hypocrisy was on show".
"The tunnel has been shown to be dud and there seems to be big questions to be asked about traffic modelling and economic benefits of the project," Mr Smith said.
"The economic benefits don't stand up according to their own hand picked expert and the public wants to know why."
In early 2015 the Victorian government announced plans for a new toll road connecting the West Gate Freeway with CityLink in partnership with Transurban.
As part of the deal Transurban will collect tolls for 10 extra years.
Construction is expected to start early 2018 and be completed by 2022.
A Transurban spokeswoman said questions about the tunnel should be directed to the government.