One Tree Island is a coral cay of about 4 hectares, situated at the seaward (southeast) end of its reef which is 3 miles long and up to 2 miles in size. The reef is an excellent example the rich development characteristic of the southern part of the southern part of the Great Barrier Reef.

The island has not been severely modified by human influence and continues to be valuable for terrestrial studies of a coral island. It is a rookery for six species of terns and a home to northern hemisphere waders.