Motorists are warned to expect delays on the Auckland Harbour Bridge this afternoon after a serious crash involving a truck and a motorbike.

Emergency services were called to the scene shortly after 1:40 pm today, following reports of a collision on the southbound lanes heading towards Auckland City.

One person, believed to be the motorbike rider, was critically injured and transported to hospital.

The Serious Crash Unit is investigating the incident, and two southbound lanes remain closed.

Traffic is already building in the area, and motorists are advised to expect delays or consider postponing their journeys if possible.

The NZ Transport Agency is urging drivers to use alternative routes and to allow extra time for travel if they must use the bridge.