An Australian tourist suffered a serious eye injury when he was shot by a paint ball gun.

The man was cycling near Hamner Springs, Canterbury, on State Highway 7A on Friday evening, when he thought he'd been hit by a bird.

"He slowed, lost balance and fell off his bike," police said.

"He picked up the sunglasses that he was wearing, both lenses had come out and he noticed paint on the inside of the frames."

He was taken to Christchurch hospital for treatment.

Before the incident he had noticed a vehicle travelling towards him, which then passed after he fell off his bike.

Police said they were making enquiries about a white or light coloured van, and were checking surveillance cameras.

The man had been intending to cycle to Nelson via Hamner Springs.