In December the government released a business case for the $5.5 billion Western Distributor, a new road linking western Melbourne to the central city, proposed by Transurban under the government's market-led infrastructure proposal policy.

The business case was released with fanfare, the government seeking to contrast itself with the Napthine government's refusal to release the business case for its contentious East West link.

However, the crucial financial data that would allow Victorians to make their own assessment of the Western Distributor deal have been redacted from the Andrews government document.

The government also refused to release two reports on the Western Distributor business case by a panel of independent infrastructure experts, Kerry Schott and Tony Canavan​.

Fairfax Media can also reveal that six of 21 appendices (consultant reports) listed as being attached to the business case have not been released, including a summary of the Transurban proposal from March 2015, project costing and toll costing reports, and a financial analysis.

The government website containing the business case says that some attachments have been partially or fully redacted due to commercial sensitivity.

Government spokeswoman Sacha Fenton said the expert panel reports "cannot be released" because they are cabinet-in-confidence.

The mere fact of a document being a cabinet document does not preclude it from release. No further explanation was given.

"This is the most comprehensive and transparent planning process ever undertaken by the Victorian government," Ms Fenton said. She did not respond when asked to explain what this meant.

On the missing appendices, Ms Fenton said the government was negotiating the project with Transurban and that "the release of sensitive commercial and other information could compromise the ability to get the best possible outcome for Victorians".

On Saturday, Fairfax Media revealed that road giant Transurban could reap tens of billions of dollars tolling CityLink users for up to 12 additional years under its proposal for the Western Distributor.

The Age and actuary Ian Bell analysed the returns Transurban is likely to generate from the deal it has struck with the Andrews government including an extension on its existing CityLink tolling concession for 10 to 12 years.

The analysis indicates a 12-year extension would generate $20 billion and $30 billion (nominal) in extra toll revenue. In 2015 terms, that figure would be $3.8 billion to $5.7 billion in 2022 dollars, based on a discount rate of 8 per cent.