The Northern Motorway northbound lanes have been closed this afternoon after a serious multi-vehicle crash on State Highway 1 near Dairy Flat.

Police are reporting critical injuries. Traffic heading north is already backed up 4km.

NZ Transport Agency Waka Kotahi (NZTA) said an official detour was along Dairy Flat Highway.

Police said emergency services were responding to the crash, which involved multiple vehicles and was reported at 2.05pm.

“Initial indications suggest there have been critical injuries,” a police spokesperson said.

Traffic data from Google shows a 4km backlog after a crash on Auckland's Northern Motorway near Dairy Flat. Photo / Google

About 4pm both northbound lanes were blocked from the Oteha Valley Rd on-ramp and traffic was building.

“Motorists are advised to expect lengthy delays and asked to take an alternate route where possible.”

Northbound lanes on Auckland's Northern Motorway are closed and traffic is being detoured.

A witness said he drove past the crash site southbound and could see about a dozen emergency service vehicles at the scene, including a rescue helicopter.

NZTA said: “Reports of a crash on #SH1 northbound between Oteha Valley Rd and Bawden Rd.

“Please follow directions of response crews and expect northbound delays in the area until cleared.”

Traffic has built up on the alternate route north along East Coast Rd.

Live traffic data from Google shows the detour is bumper-to-bumper.

St John ambulance said crew attended a traffic incident in Redvale at 2.10pm.

Eight vehicles attended the incident. The number of patients remains unknown.

More to come