Cousin's large plateau extends over most of the island and is covered with indigenous mature woodland dominated by Pisonia grandis, Morinda citrifolia and Ochrosia oppositifolia. It is almost encircled by a long sandy beach (the shape and width of which changes with each monsoon) with tall Casuarina equisetifolia and Cocos nucifera trees. The southern coast of the island is, however, rocky.
A hill, rising to 69 m, occupies the centre of the southern half of the island. It is dominated by bare rock and boulders, low woodland and Pandanus balfourii, Ficus reflexa, F. lutea, Euphorbia pyrifolia shrubs, tall herbs and ferns.
An area of mangrove and small, temporary freshwater pools are also found. Most of the coconut trees have now been cut down. Nature conservation, research and ecotourism (day visits, confined to main paths only) are the only human activities.