A plan to build a railway line to Doncaster at a cost of up to $11 billion is so costly that the proposal risks being permanently shelved. But it could be rescued with two major changes, the state's public transport lobby group says.

Responding to the Napthine government's Doncaster rail study, released in March, the Public Transport Users Association argues the study's proposal to terminate the rail line at the Doncaster Park and Ride bus interchange should be scrapped.

'We say [the tunnel] is not necessary because Doncaster rail could be realised through other means.' - Tony Morton.

The line should end instead at Westfield Doncaster, one of Melbourne's largest shopping centres and an area earmarked for future growth, the association says.

The study's related plan to build a brand new rail tunnel from Northcote to Flagstaff in the city should also be rejected in favour of a much cheaper investment in new high-speed signalling that would allow trains to run closer together.