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".
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.