The $1.3 billion widening of CityLink and the Tullamarine Freeway between Melbourne Airport and CityLink's tunnels was approved this year as a combined government-Transurban project.

The plan to widen one of Melbourne's busiest freeways and boost its capacity by 30 per cent was pitched by CityLink owner Transurban in 2012 and ultimately approved by the Andrews government in April. It had also been approved by the former Napthine government, but was put on hold after Labor scrapped the East West Link.

Transurban will spend $850 million widening the toll road between Bulla Road and the tunnels, including Bolte Bridge, in exchange for a one-year extension to its concession deed with the state and higher tolls for heavy vehicles. It is expected the company will ultimately reap $3.2 billion in extra toll revenue from the deal.

The state and federal governments will widen the Tullamarine Freeway between Bulla Road and the airport at a cost of $250 million.

Construction is due to start in October and end in 2018.

Auditor-General John Doyle's report, tabled in Parliament on Wednesday, casts doubt on the rigour of the government's "high value, high risk" framework for assessing the value for money of recent major projects pitched as public-private partnerships.

Mr Doyle said the Department of Treasury and Finance had given the road project a green light with little evidence the way it will be paid for provides best value for money for the state.

He also found the mooted safety benefits – a reduction in crashes of up to 20 per cent – were not proven.

"In our view the information provided in the business case about the potential impacts of the project on road safety is inadequate," Mr Doyle wrote.

Emergency lanes used by ambulances and fire trucks, which also act as a refuge for broken-down cars, will be removed to widen the road. The report notes VicRoads has advised using a managed motorway system to make those lanes available for emergency use outside the peak.

Victorian Greens leader Greg Barber said the true cost to Victorians of widening CityLink and the Tullamarine Freeway far exceeded $1.3 billion.

"The government keeps signing off on billions of dollars of projects without evidence we will get the benefits they are claiming," Mr Barber said.

"For the billions in extra tolls that will now go to Transurban, we could be on our way to getting a rail link to the airport."

Mr Doyle found the department's assessment of another unsolicited proposal, the $2.5 billion Cranbourne-Pakenham rail corridor upgrade, was more rigorous.

Metro proposed upgrading the chronically overcrowded rail line, by removing level crossings and buying high-capacity trains, to deliver a 30 per cent capacity boost, in exchange for availability payments of up to $5.2 billion.

The proposal was ultimately rejected by the Andrews government, which is pursuing its own, more extensive upgrade of the line.