In April, Premier Daniel Andrews revealed that his government was working with Transurban on the 2.3-kilometre tunnel and elevated freeway.

The new toll road would connect the West Gate Freeway to CityLink via a new northern road.

Transurban would keep all tolls from the new road; they have been estimated to be around $3 per car and $13 for trucks.

In return for building the new road, Transurban would extend its right to charge tolls on CityLink for 10 to 15 years.

Officers at the Department of Treasury and Finance have been assessing Transurban's proposal since at least February.

A project team within Treasury, including roads planning veteran Murray Cullinan and Ernst & Young consultant Tony Canavan – who led the 2008 Eddington transport study – are among those involved in assessing the plan.

Transurban's plan is not yet official government policy, because it is still to pass a crucial assessment process established by Treasurer Tim Pallas.

The community consultation process for the road is being done by Transurban, and not the government.

Transurban began staging information sessions for the public last week. Another is to be held on Thursday afternoon in West Melbourne.

Those who attended a Yarraville information session last week were told soil testing had not been completed on the land where Transurban wants to build the toll road's two tunnels.

A spokeswoman for Transurban said that there was a significant amount of geological data already available for the area where the proposed tunnel would be built.

"Transurban and its expert consultants have reviewed this existing geological data and are confident the ground conditions are suitable for tunnelling safely," she said.

"Specific geological testing" would start shortly, she said, "to further assist with developing the design and construction methodology. We will let local residents know before any investigative work begins".

Fact sheets on the project released by Transurban last week said that tunnelling for the project would happen "15-20 metres underground [or] around six or seven storeys underground".

They said that there would be three lanes of traffic in each tunnel.

The plan would also see Transurban widen the West Gate Freeway, which is likely to create two years of traffic issues if the project goes ahead.

Construction crews would need to shut down parts of the freeway to add two new lanes on each side, on the inside of the road. Emergency lanes would also be removed.

There will be two vent stacks built as part of the project, to funnel car and truck fumes out of the tunnel. Residents were told last week they would likely be built at either end of the tunnels.

A spokesman for the Treasurer said Transurban's proposal was being assessed "in accordance with the government's Market-Led Proposal Guidelines".

He said all appropriate measures were being taken to ensure proper probity on the project.

Greens MP Colleen Hartland, whose upper house region covers the western suburbs, said there were deep concerns about whether the government had done enough to make sure taxpayers weren't ripped off by Transurban.

She questioned whether Transurban's plans were "value for money or gold plated".

And the opposition said the scant detail available publicly about the project was concerning.

The lack of detail released by the government showed "Daniel Andrews is all talk and no action", roads spokesman Ryan Smith said.

He said the government needed to "put Victorians' interests first and not Transurban's".