Constable Terry Wood, of Queenstown, said the 27-year-old man had life-threatening head injuries after he fell off his bike on Moke Lake Rd just before 6pm.
Const Wood did not know what caused the crash.
The man, who was biking with a friend, was wearing a helmet, he said.
The crash happened on a relatively straight, downhill section of gravel road.
The injured man was taken to Lakes District Hospital and transferred to Dunedin by the Otago Rescue Helicopter Trust helicopter.