The cost of delays on Melbourne's road network is expected to triple to $9 billion by 2031 without greater investment to reduce traffic congestion, a national infrastructure audit finds.

Infrastructure Australia's report identifies the top 10 roads and freeways in Melbourne where congestion had the biggest impact on economic productivity in 2011.

The section of road between the end of the Eastern Freeway and CityLink was the costliest, on a per-kilometre basis, where the East West Link was to be built.

The Eastern Freeway was second on the list.

The Hume Freeway corridor north of the CBD is predicted to be the costliest by 2031, followed by the Western and Metropolitan Ring Road.

By then, the Federal Government's infrastructure adviser predicts the route for the first stage of the dumped East West Link will be the fifth most costly source of congestion.

The report also warned demand on Melbourne's public transport network was forecast to grow by 89 per cent in the next 15 years, with the Craigieburn, Sunshine, Werribee and Dandenong lines expected to reach "crush capacity".

Premier Daniel Andrews said the audit showed the State Government was investing in the right place.

"I think it pretty well confirms the priorities that our Government laid out before the election," he said.

"All of these things have to be looked at, not in isolation, they're part of a broader plan.

"I think today's Infrastructure Australia report, their commentary aligns very well with not our comments, but our action."

East West Link best solution for congestion: Opposition

But Opposition spokesman Ryan Smith said the Andrews Government had ditched the best way to deal with Melbourne's traffic congestion.

"The East West Link was the project to deal with much of the congestion issues that we're seeing on our roads today," he said.

The RACV's Brian Negus said the gridlock threatened to drag down productivity and living standards.

"Clearly we need to do something about roads and public transport, otherwise our liveability and our industrial development and our economic development will be threatened," he said.

Mr Negus said public-private partnerships were needed to fund transport infrastructure in Victoria, along with road-user charges.

"We need to start the debate on how a road-user charging system will actually work and how the community will embrace it," he said.

"We need to move away from the current motoring taxation system, to one where you look at charging for the proper road use, and hypothecating the revenue to fix the problem."

Public Transport User's Association spokesman Daniel Bowen said Victoria would suffer because of a lack of planning to cope with a forecast jump in Melbourne's population to six million in 2031.

"We've seen decades and decades of building freeways, very little put into public transport, no new rail lines built for many decades, and the result is people are stuck in their cars and stuck in traffic," he said.

"The only way to tackle traffic congestion ultimately is to get people out of their cars and make sure they have an alternative, make sure they have viable public transport."

Mr Bowen said the Melbourne Metro Rail project would help to reduce congestion in the inner-city, but that had to be matched with fast and frequent train services in the suburbs.