Congestion is beginning to clear on Auckland's Southwestern Motorway after a van and trailer rolled, causing a backlog of traffic.

The crash happened shortly after 12.30pm on the southbound lanes between Hillsborough Rd and Queenstown Rd.

It was not known whether the occupant of the van has suffered any injuries.

Police are warning motorists the crash will likely cause a "significant backlog" of traffic.

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The left land heading south is closed due to the vehicle blocking part of the road.