Congestion on the road is set to become so severe that Melbourne Airport has told the state government it "risks slowing Victoria's economic growth in the medium term".

The warning flies in the face of a decision by the government's top infrastructure adviser to effectively place the much-discussed rail link on the back-burner.

In its draft 30-year strategy, Infrastructure Victoria said the link would not be needed for up to three decades, suggesting the Sky Bus service could fill the gap in the interim.

But just days before the release of Infrastructure Victoria's final strategy, its predictions about the ability of the Tullamarine Freeway to handle traffic have been directly challenged by Melbourne Airport.

In a submission, it warns that even after factoring in the current project to widen the road, the Tullamarine Freeway will hit maximum peak-hour capacity in the late 2020s - about a decade earlier than Infrastructure Victoria's prediction.

It warns that dedicated lanes for buses, which crowd other motorists, are "likely to further exacerbate this issue".

The airport predicts it will handle 64 million passengers a year by 2033 - about 30 per cent more than the official Infrastructure Victoria prediction. The freeway is set to become so congested, travel times to the airport from the city are expected to rise by 50 per cent within 15 years.

"The existing road network will exceed its capacity and create untenable delays for traffic between the airport and the CBD in the next 10-15 years," the submission says.

"Constraint of the ground access routes connecting Victoria's international aviation gateway and the rest of the state risks slowing Victoria's economic growth in the medium term," it says.

It warned the project was so complex the state government would need to start planning for the link within the next 12 months "to ensure a timely delivery of the project".

The warning will place further pressure on the Andrews government to revisit the idea, despite an already crowded infrastructure agenda that includes the removal of dozens of level crossing, the Melbourne Metro Rail project and the Western Distributor road project.

Shortly before the 2014 state election the former Napthine government announced plans to build a link from Southern Cross Station via Sunshine station, branching off along the existing Albion-Jacana freight line.

The idea was put on ice after the election, with claims journey times of at least 25 minutes under that route would not be competitive with the bus, cars or taxis. But since then mounting concerns about the ability of the Tullamarine Freeway to cope has placed the idea back on the agenda.

This time, however, a northern route that would add capacity to the existing rail network, possibly running via Flemington, is now seen by many as a better, faster option.

Public Transport Minister Jacinta Allan said an airport rail link could not be build without the Metro Rail Tunnel first being completed.

"In less than two years we've fully funded and started building the Metro Tunnel, and we're getting on with the other major projects our network needs – 50 level crossing removals, bigger trains, high-capacity signalling and major line upgrades in Melbourne and regional Victoria," Ms Allan said.

Lord mayor Robert Doyle said he believed the link was "absolutely crucial".

"I don't see an alternative as that freeway fills up," he said. "I really support the Tulla widening, but that's building for the next five to 10 years. If we don't maximise that piece of infrastructure then we are losing competitiveness as we become Australia's largest city."