A CONGESTION tax to discourage motorists from driving at peak times could be introduced under a controversial plan flagged by the federal government's chief infrastructure adviser.

Infrastructure Australia chairman Sir Rod Eddington said it was time for ''mature and dispassionate'' discussion over a new system of road charges to cut congestion and help pay for major transport projects.

''Transport shortfalls across the freight and passenger networks are imposing substantial productivity constraints on our nation,'' Sir Rod told the Infrastructure Partnerships Australia conference. ''Here in Melbourne, road congestion will probably shred something like $4 billion from this state's economy this year.''

He said roads were often treated as free goods because the ''current opaque charging structure'' clouded the actual cost of using them. ''The current approach does not and cannot adequately reflect the time and location of road use,'' he said.