On Monday, the government's star traffic expert appeared before hearings into the impacts of the proposed $5.5 billion toll road through Melbourne's west.

Tim Veitch is a director of transport modellers Veitch Lister Consulting, employed by the government to assess the West Gate Tunnel. The firm - which is currently contracted to do more than $3 million worth of traffic modelling for the government - is highly supportive of the proposed road.

Mr Veitch on Monday was put under cross examination by senior barrister Nick Tweedie SC, who is acting for Melbourne City Council during the six-week environmental effects hearing.

The council is opposed to aspects of the road, which includes new city off-ramps that will put a projected 9000 extra cars a day into West and North Melbourne.

Under questioning by Mr Tweedie, Mr Veitch was forced to admit that an internationally respected traffic expert hired by the Andrews government to review his firm's traffic predictions for the road had refused to support those conclusions.

He is the second transport expert hired by the government to dispute the the traffic modelling behind the proposed road project.

Last week, The Age revealed that long-time Victorian government transport consultant William McDougall had similar concerns about the road.

The accuracy of traffic predictions used for massive toll road projects like the West Gate Tunnel is crucial; overly optimistic projections of expected vehicle numbers will dramatically inflate the likely benefit-to-cost ratio used to justify a project's construction.

The government says that the benefit the West Gate Tunnel will return to the Victorian economy is $1.30 for every $1 spent on it - but this figure is highly dependent on traffic projections.

Mr Veitch said that he had "an ongoing difference of opinion" with New Zealand traffic expert John Allard over the validity of projections by Veitch Lister Consulting that overwhelmingly supported the toll road.

Mr Allard, who has declined to discuss his confidential findings with The Age, was appointed by the government to provide an independent review of Veitch Lister's traffic modelling for the toll road.

Mr Veitch on Monday said that Mr Allard was "well regarded" as a transport and traffic expert.

And he agreed that Mr Allard had disputed the projected traffic levels predicted by Veitch Lister, which the Andrews government has relied on to back the road.

Mr Veitch's company has made traffic forecasts for many Australian toll roads, and its traffic projections were used to support the East West Link - the controversial toll road dumped by Premier Daniel Andrews at a cost of more than $1 billion.

Victorian Greens MP Colleen Hartland represents the western suburbs, and along with fellow Greens senator Janet Rice, has made repeated attempts to get the Victorian government to release Mr Allard's report.

Both Ms Hartland and Ms Rice said it was now more important than ever that the report was released. "The fact they are not coming clean makes it look like they have got something to hide," Senator Rice said.

Ms Hartland said the West Gate Tunnel increasingly looked more like it was "about making profits for Transurban" than it was "getting the people of the west to work or getting trucks off streets".

Roads Minister Luke Donnellan said last week, in response to questions emerging over the reliability of traffic modelling for the project, that "a large group of traffic experts" had agreed the road stacked up.

"An assessment by 16 different traffic modellers," Mr Donnellan said, had found that "for every dollar you spend, you get $1.30 return".

But his office on Monday declined to release John Allard's independent review of the traffic figures behind the toll road proposal. The report was "cabinet-in-confidence and cannot be released", a spokesman for the minister said.

The government argues the project will "produce travel time savings of up to 20 minutes between Melbourne and the western suburbs, Geelong and Ballarat".