One Tree Island is a small coral cay. It is located near the Tropic of Capricorn in the southern Great Barrier Reef, 96 km due east nor east of Gladstone, Queensland, Australia, and 450 km north of the state capital Brisbane. The island is part of the Great Barrier Reef chain of islands, and is part of the Capricorn and Bunker Group of island and forms part of the Capricornia Cays National Park.