Some of Melbourne’s busiest railway lines are in urgent need of capacity upgrades, Infrastructure Australia has said, warning that public transport congestion in the city will hit the Australian economy if left to worsen.

The national advisory body has pinpointed several rail lines, including the Hurstbridge, Mernda, Melton, Craigieburn and Cranbourne lines, as potential barriers to economic productivity, with its projections showing peak-hour demand is on course to outstrip capacity.

The $600 million Mernda rail extension opened just six months ago but is already under pressure.

Rail congestion is rising as big population increases in the outer suburbs combine with job concentration in the inner city.

The congestion threatened to leave Melbourne commuters stranded on platforms and force them back into their cars, Infrastructure Australia warned.