Althorpe Island is the wild outpost of Yorke Peninsula diving. Rising steeply from the sea at the mouth of Spencer Gulf, with a lighthouse first lit in 1879, it is surrounded by walls, reefs and boulder slopes, and the SDFSA describes more than 50 great dive sites around the island, with walls over 20 m, reefs, wrecks and seal colonies to 40 m. The Marine Life Society notes that the south-side cliffs drop straight to a rocky bottom at about 25 m, with more seals and larger blue groper than Haystack Island.

The water here is oceanic and often clear, and the marine life reflects the exposure. Walls and caves carry sponges, gorgonians and soft corals, and the fish life is prolific: big western blue groper, schools of sweep, bullseyes, boarfish, blue devils, snapper and trevally. Australian sea lions and New Zealand (long-nosed) fur seals haul out on the island and join divers in the water, especially at Seal Cove, where the Marine Life Society describes a population of over 150 playful seals.

Crayfish and abalone are present in season. The island is also a shipwreck site. The Investigator Strait Shipwreck Trail lists the SS Pareora (1919) and Mylor Star (1982) as dive sites here, plus several vessels not yet located – Young St George (1878), Welling (1892), Rapid (1937) and Altair (1971).

Althorpe is an advanced destination: remote, current-swept, weather-exposed and deep in places. It needs a capable boat, a careful forecast and experienced divers. For those who get there on a good day, it offers some of the best temperate diving in South Australia outside Kangaroo Island.

From Adelaide drive about 300 km (3.5 hours) via Port Wakefield, Minlaton and Warooka to Marion Bay; Dhilba Guuranda-Innes National Park begins just west of the township (vehicle entry fee, pay online or at the Stenhouse Bay visitor centre). Launch at the Marion Bay ramp and run about 20 km south-west past Cape Spencer (40-50 minutes in a fast boat), or from Pondalowie Bay in settled weather. The crossing is open water exposed to Southern Ocean swell, so it needs a seaworthy boat, good forecasts and experienced skippers.

Landing on Althorpe Island (a conservation park) is not permitted without authorisation – the Friends of Althorpe Islands manage the lighthouse station.

The southern tip of Yorke Peninsula faces the open Southern Ocean and Investigator Strait, so south-westerly and southerly swell is the controlling factor: most exposed sites need swell under about 1.5 m and are best after several days of northerly or north-easterly winds, which blow offshore along this coast. Winter cold fronts bring big south-westerly swell and strong winds and can close the area for days. Summer afternoon south-easterly sea breezes make boat runs rough, so launch early.

Visibility is typically 10-20 m in settled conditions but drops sharply with swell-driven surge. Althorpe is very weather-exposed, with strong currents at times; it should only be attempted with swell under about 1.5 m, light winds and a stable forecast for the return crossing.

Choose a site in the lee of the prevailing swell: the south-side cliffs for walls dropping to 25 m, or Seal Cove to finish a dive with seals. Descend on the wall, work along at your planned depth, then shallow up the wall to finish with seals and groper in the shallows. Carry an SMB for drift pick-up.

Deep walls allow 30-40 m dives – nitrox and conservative profiles recommended given distance from the chamber.

No regular commercial operator; occasional Adelaide shop and club expeditions with capable boats, and private boats from Marion Bay. Local charter vessels have historically visited sites such as CD's Bommie.