Traffic remained heavy on a Sydney motorway several hours after a truck jack-knifed on Tuesday, leaving traffic queued for 11 kilometres.

All northbound lanes of the M7 were closed at 1.45pm and southbound lanes had just reopened after the truck crashed at Prestons, near Liverpool, about 1pm.

Photos from the scene showed the truck managed to diagonally wedge itself across three lanes of the bridge, with the cab facing in the opposite direction.

Motorists heading south had more than an hour added to their journey.