Police are responding to an incident on the Northwestern Motorway where a woman sustained serious injuries.

It is understood the woman may have been injured by an object flung from another car, just after the Royal Road on-ramp, city-bound.

A small hole can be seen in the bottom corner of the windscreen of a grey car the woman was believed to have been travelling in.

The car the woman was in has a small hole in the bottom corner of the windscreen.
The car the woman was in has a small hole in the bottom corner of the windscreen.

Residents from houses that back on to the edge of the motorway said they heard a loud bang with one man saying "I thought it was a gunshot" and another neighbour saying "it sounded like a loud crack".

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It is still unclear what caused the small hole in the passenger side of the front window-shield of the car.

Police are yet to confirm exactly what has happened. They responded shortly before 10am.

St John said one person sustained serious injuries in the incident and had been transported to Auckland City Hospital after being treated at the scene.

They sent two ambulances to the scene, after being called at 9.41am today.

Police are investigating. Photo / Elizabeth Binning
Police are investigating. Photo / Elizabeth Binning

People walking in the area said they heard sirens and two ambulances rushing to the scene.

Three police vehicles, one police motorcycle and an unmarked car remain at the scene.

The silver car has stopped inside cones blocking off road works.

From where the car has stopped it would appear it had been driving towards the city rather than had come on from the Royal Rd on-ramp.

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Police are at the scene making inquiries into the circumstances. A reporter at the scene said an unmarked police car was there.

A police helicopter was also seen hovering above.