State Highway 2 between Wellington and Lower Hutt is now clear after a van rolled in the northbound lanes.
Traffic was brought to a standstill shortly before midday when the crash happened just north of the Ngauranga gorge.
The driver of the van had to be cut from the vehicle.
A Wellington Free Ambulance spokeswoman said one person had been taken to Hutt Hospital in a serious condition.
Another person was treated at the scene with moderate injuries.
