Updated 3.23: Six people have been treated for injuries after a seven-vehicle crash on the Auckland Harbour Bridge.

Cars trying to avoid an abandoned plank of wood caused the nose-to-tail pile up, just after one o'clock.

St John, which sent two vehicles to the scene, says the patients' injuries ranged from minor to moderate.

No one was taken to hospital.

Two of the city-bound lanes were closed while emergency services worked to clear the crash site.

Police will be reviewing CCTV footage to find out where the 1.2 metre piece of wood came from.