St John paramedics and their patients were put in danger by a "senseless act" on Saturday night when a bottle flung from an oncoming car struck an ambulance on the Parapara Rd.
The ambulance was taking two patients from Turoa skifield to Wanganui Hospital about 5.30pm and was 10 minutes south of Raetihi when one of two or three youths in the car threw the large bottle, St John districts operations manager Ian May said.
The bottle hit the ambulance grille, bounced on to the windscreen and smashed, covering the glass with a sticky liquid.
Mr May said the driver got a "large fright" but managed to bring the ambulance to a safe stop.
"There's significant damage to the front of the vehicle that will be costly," he said, adding that fortunately there was nothing mechanically wrong with the vehicle and it was able to continue the trip to hospital with the patients, described as in "moderate" condition.