[1] With an area of 28 hectares (69 acres) the island is made up mostly of granite.

Coffin Island forms part of the Two Peoples Bay and Mount Manypeaks Important Bird Area, identified as such by BirdLife International because of its significance in the conservation of several rare and threatened bird species.

[3] Other seabirds such as flesh-footed shearwaters and little penguins also have colonies on the island.

Australian sea lions also make use of the island in smaller numbers.

The island has sufficient soil accumulated over its surface to support Berry Saltbush heathland.