Police have closed the northbound lanes of Auckland's Southwestern Motorway.

It is unclear what is happening as police would only say they are responding to an "incident".

The motorway was closed at Coronation Rd, a spokeswoman said.

A witness has told the Herald a backlog of traffic behind the closure is being diverted to a side street.

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Police, St John and Fire Emergency New Zealand are on the scene.

Motorist were warned to expect delays.

- More to come