Two people have died in a serious crash that closed Wellington's Terrace Tunnel to traffic in both directions.

The crash happened at the northern end of the city's Terrace Tunnel on State Highway One - the main route in to and out of Wellington.

The tunnel re-opened at around 4.30pm, after having been shut since about 1pm.

Police have now confirmed that a man and a woman riding on a motorcycle have died, but no other details have been released.