All lanes are open on State Highway 1 out of Wellington after an oil spill in the Ngauranga Gorge cut northbound traffic to one lane.
But the Transport Agency said motorists could expect lengthy delays throughout Monday evening, because motorway traffic had backed up as far as the Basin Reserve.
The spill occurred when a truck lost its sump as it headed up the Ngauranga Gorge at about 2.30pm.
The Transport Agency's Mark Owen says the oil combined with heavy rain caused big problems. One car spun out of control and trucks struggled to get enough traction to climb up the Gorge.