Victorian Treasurer Tim Pallas has hosed down expectations for a high-speed multibillion-dollar airport rail tunnel, despite repeated pressure from the Morrison government to sign up for one.

Plans for a high-speed rail tunnel to the airport are being debated. Credit:Andrew Quilty

At a business event in Sunshine, Mr Pallas raised the possibility of a cheaper, above-ground route that would stop at suburban stations, according to people in the room.

The Treasurer said the government was considering delaying building twin rail tunnels between the CBD and Sunshine, warning that the project's cost could skyrocket and even double if the tunnels were built, the sources said.

The state government was concerned about sluggish patronage on the line in the first few years and was considering having trains stop at suburban stations, Mr Pallas reportedly told the Delphi Bank event on Tuesday.