Two people have been injured in a crash which has closed an Auckland motorway this afternoon.

Initial reports indicate a northbound car struck the median barrier on the southwestern motorway about 11.50am and rolled over, police said.

A female driver suffered serious head injuries, and a passenger suffered less serious injuries. They were taken to hospital by ambulance.

The motorway's northbound lanes between the Puhinui and Massey Rd interchanges, was closed following the crash, police said.

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Northbound traffic is being directed off the motorway at Cavendish Rd to avoid the scene. The Puhinui Rd on-ramp is closed.

The motorway is expected to be closed for several hours while the wreckage is cleared.

Drivers are advised to avoid the area if possible, or take extra caution.

The police serious crash unit will investigate.