Think Melbourne's trains, trams and buses are busy now? Just wait.

The use of public transport in Melbourne – and heavy rail in particular – will soar over the next two decades, according to a new audit of the nation's transport needs by the federal government's infrastructure adviser.

"Demand for public transport will increase by 89 per cent by 2031," it predicts, indicating governments will have to focus on expanding the capacity of existing services, and provide new infrastructure to growth areas on the outskirts of cities.

"Crush loads" – the term used by transport operators to describe a full service – are to be felt worst on four metropolitan rail lines: Craigieburn, Sunshine, Werribee and Dandenong.