Congestion is easing on State Highway 1/Christchurch Northern Motorway after the clearance of mud, rocks and other debris spilled on and near the Waimakariri River Bridge.

Police were called out around 3.40pm today as the spilled material was impacting northbound traffic, leading to substantial traffic queues.

Contractors for New Zealand Transport Agency Waka Kotahi were dispatched to clean up using equipment such as a sweeper truck, and the road is now reported to be clear.

Road users may still encounter congestion and are advised to travel with care in the area.