A truck carrying a large amount of gravel has rolled on one of the main routes to Auckland Airport.
The crash happened in the southbound lanes on State Highway 20, the Southwestern Motorway, near the Portage Road overbridge about 4.20pm.
Gravel covered the road.
Motorists were asked to avoid the area, or use State Highway 1 as an alternative route.
Emergency services are at the scene and traffic management contractors worked to clear the road and get traffic moving again, police said.
NZTA tweeted that the southbound lanes were now clear, though congestion remained.
Auckland Airport tweeted "traffic is now flowing through all airport roads and congestion has eased".