Four people have been injured in a three-car crash, which is backing up traffic on the Southwestern Motorway in Auckland.

The crash, which happened just after 8am, is blocking the right-lane northbound after Puhinui Rd, Manukau.

One of the cars was tipped on its side said a police spokeswoman.

A St John's spokesperson said three people have moderate injuries and one has minor injuries.

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NZTA have confirmed the crash is now clear of the lanes but the motorway continues to be congested from the Southern Link.