Only one of four proposed routes for Melbourne's "missing link" freeway has a price tag within the Victorian government's $10 billion cost estimate.
The other three range in price from $16 billion to $23 billion and each includes more than 13km of expensive tunnels.
Premier Daniel Andrews on Monday revealed four possible routes for the North East Link, which will complete the Metropolitan Ring Road by connecting to either the Eastern Freeway or EastLink.
The freeway link is Victoria's most important infrastructure project, the premier said, but he declined to reveal his preference among the route options.
"This really is the number one priority," he told reporters.
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"It's a bit early for us to talk about final cost, we believe this can be brought in at about $10 billion."
The first option is the most direct route, putting up to four kilometres of tunnels under Heidelberg at an estimated cost of at least $6 billion.
The other three options are longer, would involve much more tunnelling, and cost up to $23 billion.
They are also steeper, potentially causing problems for trucks.
The direct route would run through the electorate of Opposition Leader Matthew Guy, who said it would funnel 100,000 cars on to the Eastern Freeway and into Melbourne's suburban inner north.
"You'll just create a northeast car park," he told reporters.
Mr Guy said a connection was "absolutely needed" but it would be a "dud of a project" if it did not include an East West Link to keep traffic moving at the end of the Eastern Freeway.
Labor axed the East West Link tunnel project when it took power in 2014.
The corridors will involve property acquisitions, crossing the Yarra River and upgrading existing roads.
The Greens called for the new road project to be dumped altogether.
"If the government actually asked the voters, they'd say 'invest in public transport', like the Doncaster rail line," Greens Eastern Metropolitan MP Samantha Dunn said.
A decision on the preferred route is expected to be made by the end of 2017, and Labor has previously promised to take the project to voters at next year's state election.
