Cartier Island is an uninhabited and unvegetated sand cay in a platform reef in the Timor Sea north of Australia and south of Indonesia. According to the Commonwealth of Nations, the land area of the Cartier Islands is 0.004 square kilometers (0.4 hectares). It is located at 12°31'S 123°33'E, on the edge of the Sahul Shelf, about 300 kilometres off the north west coast of Western Australia, 200 kilometres south of the Indonesian island of Roti, and 70 kilometres south-east of Ashmore Reef.