An Australian tourist cycling to Nelson on Friday evening was hit in the eye by a paint ball fired from a passing van.
He lost balance and fell off the bike, about 4km out of Hanmer Springs.
The incident happened at 6.10pm on State Highway 7A.
Police said the man at first thought he had been hit by a bird.
But he noticed paint on the frames of his sunglasses after both lenses came out and saw a white van driving away.
He was taken to Christchurch hospital with a serious eye injury.
Police want to hear from anyone in the area who saw a white or light-coloured van.