A multi-car smash on Auckland's Southwestern motorway is causing a major traffic pile-up, blocking two of three southbound lanes just after the Mangere Bridge.

The crash happened around 9.40pm near the Rimu Rd overbridge in Mangere.

A police spokesperson said several cars appeared to be involved in the crash. Callers said two of the cars were facing the wrong way.

According to Auckland's Joint Transport Operations Centre (JTOC), the centre and right-hand lanes are blocked.

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Ambulance and fire crews are also at the scene. No diversions are in place yet. It is not known if anyone is injured.