The business case for Melbourne's West Gate Tunnel project is based on flawed traffic modelling and cost-benefit analysis work, a transport planning consultant who previously advised the Victorian Government says.

William McDougall has made a series of strong criticisms in a submission to a Senate inquiry into existing and proposed toll roads in Australia.

West Gate Tunnel Project: New tunnels from West Gate Freeway under Yarraville

New tunnels from West Gate Freeway under Yarraville Widening of West Gate Freeway from eight to 12 lanes

Widening of West Gate Freeway from eight to 12 lanes 14km of new and upgraded cycling and walking paths

14km of new and upgraded cycling and walking paths New bridge over Maribyrnong River

New bridge over Maribyrnong River New elevated road along Footscray Rd

New elevated road along Footscray Rd Better access to ports, removing trucks from residential areas

Better access to ports, removing trucks from residential areas To be completed by 2022

Mr McDougall was part of an internal Government peer review of the transport modelling and cost-benefit analysis used for the $5.5 billion West Gate Tunnel project, which was first known as the Western Distributor.

Under the plan, twin tunnels will be built under the Maribyrnong River to connect the western suburbs with the city. It is designed to provide an alternative to the heavily used West Gate Bridge, which will also be widened, and reduce truck congestion in suburban areas.

But Mr McDougall has raised serious questions about the way the business case for the project was put together.

"I consider that, from my direct involvement as a peer reviewer, the justification for this project is based on flawed traffic modelling and cost-benefit analysis," he wrote.

"This and other experiences through my career strengthens my opinion that the work done to get a toll road project funded often lacks rigour and thoroughness, and is frequently biased."

Mr McDougall wrote he was "surprised" that the consultants on the tunnel project were the same used in the East West Link (EWL) business case, which was dumped by the Andrews Government, amid much fanfare, after it was elected.

"There had been many concerns and problems expressed with the business case for the EWL, including the transport modelling and cost-benefit analysis," Mr McDougall wrote.

'Illogical' traffic modelling used for West Gate Tunnel

The tunnel will provide the western suburbs with a second river crossing. ( Victorian Government )

Last month, a report prepared by a council officer for the City of Melbourne labelled the tunnel project "a dog's breakfast" and even called for it to be scrapped.

Mr McDougall said the transport modelling was based on traffic data that was significantly higher than recent travel surveys, and used an untested model for future forecasting instead of the Government's own forecasting model.

"It is a methodological 'fudge' rather than a real attempt to improve the modelling process," Mr McDougall said.

He said the "illogical" model overestimated the number of projected car trips and kilometres travelled for the the proposed road, and used a wider area to boost the population numbers used in the business case.

Mr McDougall said he was removed from the review of the project without explanation after raising what he said were serious technical and cost-benefit concerns with Treasurer Tim Pallas.

Roads Minister Luke Donnellan has previously said some people working on the project were only contracted for a limited period of time.

Today, Mr Donnellan rejected suggestions the traffic forecasts and economic benefits had been deliberately distorted and misrepresented.

"I certainly haven't deliberately misrepresented anything," he said.

"These are assessments that a large group of traffic engineers undertook and it's appropriate that there is discussion, dissent and to come up with a consensus figure. [That] is very much what they've done."

Mr Donnellan said the West Gate Tunnel Project would return $1.30 for every $1 invested.

Grattan Institute questions 'secrecy' around project

The project will also include elevated cycling paths and veloways. ( Supplied: Victorian Government )

Mr McDougall was involved in bidding processes for CityLink and EastLink and said, in his experience, those trying to justify major transport projects like toll roads created biased business cases that produced "bullish forecasts and optimistic economic and financial projections". He said they would not stand up to independent review or scrutiny.

"Very few project justifications have been undertaken with the transparency, objectivity and completeness which is required," he said, writing:

"The Western Distributor project is a strong case in point. My direct experience as a reviewer confirmed my long-held suspicions of substantial 'optimism bias' in the appraisal process. 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. This was done on what was supposed to have been an impartial assessment of the project from a Government perspective, rather than a proposal by a private proponent or tenderer."

In another submission to the inquiry, Melbourne based think-tank Grattan Institute also raised questions about the secrecy surrounding the project.

"A lack of transparency gives rise to suspicion that the deal may not be in the best interests of the public," it said.

It also said the risk of cost overruns on construction and operations, which would be covered by the taxpayer, was understated.

The Grattan Institute said funding the project through private finance, via tolls and general revenue, was more expensive than government borrowing.