The island is a
nature reserve (declared in 1983) and managed by the Department of Parks and Wildlife; its reserve number is 30049. The island is also registered as a national estate. The island is 91.9 hectares (227 acres) in area.
Michaelmas Island has steep rocky shores with submerged reefs all along its northern southern and eastern sides in depths up to 30 metres with large
granite boulders. The faces are covered in a diverse range in corals and sponges and
home to a wide variety of marine life. A whale chaser “Cheynes III” that was scuttled in 1982 is located off the western side of the island and is regarded highly as a diving site. Michaelmas Island is an important habitat for both resident and migratory birds. The habitat of these species is threatened by weed invasion such as Taylorina and feral animals such as rats.