Haystack Island is a slab of rock with towering cliffs about 6 km south-west of Stenhouse Bay, part of the Althorpe Islands Conservation Park and, since 2012, surrounded by a habitat protection zone of the Southern Spencer Gulf Marine Park. It is only about 500 m long and 120 m wide but rises 43 m out of the sea, and it hosts breeding seabirds that have made it a permit-only area since 2007. Underwater, the Marine Life Society describes shallow ledges around the island at about 15 m, with crevices and ledges visited by seals, large western blue groper, southern blue devils and plenty of other fish.

The ledges are dressed in kelp on top and sponges, ascidians and gorgonians on the vertical faces, and the seals – New Zealand fur seals and Australian sea lions from the Althorpe group – often come to inspect divers. Haystack's bigger attraction is the long reef that runs parallel to the island about 500 m offshore (see Haystack Reef), and most boat trips combine the two: a deeper wall dive on the reef and a shallower seal-and-groper dive along the island. The island is very exposed and currents can be strong, so it is a calm-day site for divers with boat experience and good buoyancy.

It is closer and more sheltered than Althorpe Island, making it the usual next step for Innes divers who want wilder diving without the long crossing.

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 Marion Bay and run about 12 km west-south-west past Stenhouse Bay to Haystack Island (25-30 minutes), or from Pondalowie Bay around Cape Spencer in settled weather. Anchor in sand off the ledges on the side sheltered from the swell.

The island is a prohibited area (since 2007) where landing requires a permit to protect breeding seabirds.

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. The Marine Life Society describes Haystack Island as very exposed with strong currents possible.

Descend on the lee side at 10-12 m and follow the ledges and crevices along the island's base, watching for seals, then work up into the shallows to finish. Use an SMB – drift pick-ups are common if current picks up.

No resident operator; visited by Adelaide shop and club boat trips to Innes and by private boats from Marion Bay, usually combined with Haystack Reef.