Topgallant Island lies off the Elliston coast in the Great Australian Bight and is one of the least-dived island sites in South Australia. The Marine Life Society describes walls and gutters to about 20 m, especially on the rarely dived exposed side, with western blue groper, silver drummer, horseshoe leatherjackets, scalyfin, magpie perch and blue devils. Oceanic water around the outer islands of the west coast is often very clear, and the walls carry sponges, gorgonians and kelp, with gutters that funnel fish and swell.

Australian sea lions and New Zealand fur seals occur throughout this island group, and dolphins are common. The crossing from Elliston is the main barrier: 25 km of open water facing the Bight, possible only on a small number of calm days each year. The islands are protected, and landing is restricted to protect seabird and seal colonies.

Topgallant suits experienced, self-sufficient boat divers who can pick a weather window. For them it offers a rare chance to dive a wild, clear-water island reef where few divers have ever been. Carry full safety equipment, including a satellite beacon and oxygen, given the distance from help.

Plan the return crossing with plenty of margin, as afternoon sea breezes and building swell can make the run back to Elliston dangerous for small boats.

Launch at the Elliston boat ramp (main swimming beach) and cross west-south-west about 25 km to Topgallant Island (45-60 minutes). This is an exposed open-ocean crossing – only go in calm, settled weather with a seaworthy boat. Carry a VHF radio or satellite beacon, oxygen and spare fuel, and bring full cylinders, as air fills in Elliston are limited.

The west coast of Eyre Peninsula faces the Great Australian Bight and receives almost constant south-westerly Southern Ocean swell; open-coast sites need swell under about 1.5 m and are only diveable a handful of days a month, usually after several days of northerly or easterly winds, which blow offshore. The large sheltered bays (Coffin Bay, Waterloo Bay, Venus Bay, Baird Bay, Streaky Bay, Denial Bay) are protected from swell and can be dived in most conditions except strong onshore winds. Summer brings strong afternoon south-easterly sea breezes; winter brings cold fronts with big swell but also calm, clear spells.

Water is cool: about 14-15°C in winter and 17-21°C in summer.

Choose the lee side, descend on the walls to 15-20 m, follow the gutters, and ascend along the wall; the exposed side only on exceptional days.

No operator; private boats only.