Five people were injured when two boats collided on Lake Middleton in the South Island on Monday afternoon.
Police say one boat was towing a water skier and the other a ski biscuit when the collision occurred after 2pm.
They say the drivers failed to see each other until the last second.
A three-year-old has been flown to hospital with a head injury and a 67-year-old also needed to be checked medically.
Three others were taken to hospital by ambulance with suspected broken bones.