A serious crash south of the Waimakariri River has closed Christchurch's northern motorway.

Police southern communications Inspector Peter Hegarty says two cars collided at about 20 past five.

He could not confirm how many people were in the vehicles, but said some had serious injuries.

The southbound lane is closed, with vehicles being diverted off at the Kaiapoi off-ramp.

The Tram Rd and Kaiapoi on-ramps are also closed.

Police, ambulance, fire and the Serious Crash Unit are all at the scene.

Inspector Hegarty says traffic is flowing in the northbound lanes.

He says there has been a minor nose-to-tail crash further up the road, because of the congestion.