Sydney motorists confront seven of the eight worst road corridors in the country, according to the first national audit of Australia's infrastructure.

In a landmark inquiry into the country's transport, water, energy and telecommunications systems, the federal government's top infrastructure adviser has warned massive investment is needed to avoid a congestion crunch in the nation's biggest cities.

Heavy traffic on Victoria Road. Credit:Simon Alekna

The audit by Infrastructure Australia spotlights transport networks as needing serious attention to cope with a national population that will grow from 22.3 million in 2011 to 30.5 million in 2031.

In Sydney, Pennant Hills Road between Parramatta and Hornsby emerges as the road with the worst economic toll in the country, measured according to the cost of delays caused by congestion per road lane.