A person has fallen from an overbridge onto Auckland's Northwestern Motorway and died.

Police have closed the citybound lanes underneath the Hobsonville Road overbridge.

Traffic is being diverted at the intersection of Brigham Creek Road and Fred Taylor Drive.

Citybound traffic on SH18 is also being diverted at Brigham Creek Road.

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The Hobsonville Road overbridge is also closed to vehicles and pedestrians.

Motorists are being asked to avoid the area.

Police the incident was reported shortly before 3.15pm.

"Lane closures will be in place for some time and motorists are urged to avoid the area if possible and expect delays," police said.

St John confirmed they responded with two ambulances.

A motorist said she saw three police cars heading west near the Lincoln Rd-turn off.