A major crash on Auckland's Northern motorway has left a vehicle on its side and a second car on its roof, trapping two people.

Emergency services have rushed to the scene.

Fire crews have freed two people from the overturned vehicles using rescue equipment, Fire and Emergency NZ northern shift manager Colin Underdown said.

Police are in attendance. A spokesman said one person had suffered moderate injuries and another minor injuries. Both were being taken to hospital.

"South-bound traffic is currently reduced to one lane and there is already traffic building up in the area," the police spokesman said.

SH1 NORTHEN MWY, SOUTHBOUND - 12:30PM
A crash is blocking lanes citybound between Greville Rd and Upper Harbour Hwy. Please avoid this route or expect delays in both directions as #AklTraffic slows to pass the scene. ^TP pic.twitter.com/NkvK1nk8UC

— NZTA Akld & Nthlnd (@NZTAAkl) November 6, 2019

"Police are advising motorists to avoid the area or to delay travel, if possible, while the scene is cleared."

The crash happened just before 12.30pm on SH1 southbound, between Greville Rd and Upper Harbour Highway.

"Citybound lanes remain blocked as emergency services attend. Congestion is building in both directions. Please avoid this section of motorway -in both directions- until the scene can be cleared," the NZ Transport Agency tweeted.

"Please avoid this route or expect delays in both directions as #AklTraffic slows to pass the scene," the agency said earlier.