A car has smashed the concrete median barrier on SH2 between Wellington and Lower Hutt.
A police spokesman said a car had spun and hit the barrier before it landed in the left northbound lane in the Horokiwi area.
"One caller said there were bags of plaster that had come out the back of a vehicle and gone on the road.
"It's quite a busy time on the motorway."
He said no one is trapped.
Police are still at the scene.