Up to four people were injured in a crash that closed part of Auckland's north-western motorway this afternoon.

The NZ Transport Agency said about 12.40pm that State Highway 16 after Royal Road, west-bound, was closed just after the Royal Road on-ramp and Hobsonville Road.

Emergency services are at the scene, where a car crashed earlier.

No one is thought to have been seriously injured in the accident.

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A police spokeswoman said the crash was thought to have involved three vehicles.

All lanes reopened at 12:56pm according to a tweet from the agency, but there was still congestion.