The cost of the North East Link in Melbourne is predicted to cost $26.1 billion, more than double the original estimate, driven by the cost of materials, inflation and energy prices.
The major road project in Melbourne's east was originally tipped to cost $10 billion when it was unveiled in 2016 and priced at $15.8 billion in the 2017 business case.
Construction is already underway on the 10km long connection between the Western Ring Road and the Eastern Freeway, which is set to be as wide as 20 lanes in some sections and includes 6.5km of tunnels.
Victorian Premier Jacinta Allan attributed the $10 billion blowout to several issues including changes to the project, rising construction costs, supply chain issues and the inflationary impact of global events like the pandemic and war in Ukraine.
"The consequences of those global events has been additional investment, additional costs being added across a whole range of things and this project is no different," Ms Allan told reporters on Friday.
"The materials we need like asphalt, steel and concrete, the cost escalation for those products have gone up, also too, things like power we need to run the (tunnel boring machines)."
Treasurer Tim Pallas said about 60 per cent of the additional costs came from the "deliberate decisions" by government on design changes to enhance the project.
"That was about making sure that we weren't running roads through communities where we could avoid it,'' he said on Friday.
"We heard people wanted better noise (amendments) and that all comes at a cost, but it's also an investment for the future."
A Major Road Projects Victoria cost explainer provided by the government showed the rising cost of supplies totalled $3.1 billion.
Tweaks to the project cost $6.4 billion, including adding an extra two kilometres of tunnel, freeway upgrades, design adjustments, a rail extension and a busway.
The road is to open in 2028 and Ms Allan said congestion around Melbourne would only get worse if the road is not built.
Opposition major projects spokesman David Southwick said the price jump cast into doubt other costings in the state's budget and accused Ms Allan of mismanagement.
"How could we believe any major project that this government touches isn't going to blow out like this one," he told reporters on Friday.
"This is a legacy that no one wants, no one signed up for, and we're all paying the price."
Opposition leader John Pesutto said the government needed to be transparent with the community on how exposed Victorians are to project blowouts.
It's the latest in a string of infrastructure project blowouts since Labor came to power in the state in 2014, including the West Gate Tunnel which is $4.1 billion over budget.
Friday's state budget update tipped Victoria's net debt to reach $177.8 billion by mid-2027, some $6.4 billion higher than predicted six months and attributed to rising infrastructure investment.
