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.
The left northbound lane was blocked after the crash, causing significant delays for motorists.
The crash has now been cleared and all lanes reopened, but delays continue in both directions.