It is a sanctuary for both birds and plants, holding species that on the mainland of New Zealand are rare or have died out.
[5] The shortest distance across the water from Rakiura to Ulva Island is around 800 metres (2,600 ft), and this is within the swimming range of Norway rats.
[6] The Department of Conservation currently maintains the public portion of the island including its predator free status.
A small portion of the island (approximately 8 hectares or 20 acres between Sydney Cove and Post Office Bay) is privately owned.
The liverwort species Lophocolea mediinfrons is thought to be endemic to northeastern Ulva Island.