The pilot reported that on arrival at Dunk Island he made a normal approach for runway 14, and on final approach at a height of about 50 feet he encountered a sudden downdraft. The aircraft landed heavily on the left main gear before he could reduce the sink rate. The pilot stated that he subsequently examined the aircraft but did not notice any signs of damage. The damage was discovered after the pilot had returned to Townsville and reported that he had made a heavy landing to the operator. The landing impact had damaged the gear attachment by displacing it vertically upwards, a rib was bent, and rivets were pulled on the lower wing skin. The spar was bent but returned to its normal shape when damaged skin was removed. The pilot stated that the wind at the time of the accident was southeasterly at about 10 -15 knots, and that he had not previously landed at Dunk Island.