The Andrews government is considering a budget airport rail link that would scrap a tunnel to the city and stymie fast rail to Geelong.

The mayor of Geelong and a leading rail expert are calling on the state government to ditch the cheaper option, warning that it would make airport services slow and unreliable.

Their concerns have been echoed by federal Infrastructure Minister Alan Tudge, who insists Victoria needs a dedicated airport line from the city to ensure fast, frequent services to the airport.

When the Andrews government confirmed last year that the airport rail to Tullamarine would follow a route through Sunshine, the project was estimated to cost $8-$13 billion. The cheaper option now on the table is believed to cost over $5 billion.

The new proposal would see airport trains use existing rail lines between Southern Cross and Sunshine, and add a new line between Sunshine and the airport, sources close to the project have said.