The Aldinga Drop-off is one of Adelaide's most spectacular dives and the showpiece of the Aldinga Reef Sanctuary Zone. Less than 1.5 km off Aldinga Beach, an extensive limestone reef ends in a sheer underwater cliff that falls about 10 m, from 5-8 m on the reef top to 18-21 m on the sand below. The face is, in the Federation's words, a mass of caverns, crevices and overhangs, and the edge extends for a long way, so there is always more to explore.

The reef has been protected since 1971, first as the Aldinga Reef Aquatic Reserve and since 2012 as a sanctuary zone of Encounter Marine Park, and the Marine Life Network notes that its reserve status shows in the marine life. The wall is covered with sponges, ascidians, gorgonians and soft corals, particularly in the shaded overhangs. Fish include blue devils, boarfish, morwong, blue groper, wobbegongs, scorpionfish, sweep, pike and kingfish, and large schools of snapper are a feature at times.

Cuttlefish, octopus and stingrays are regular, and Port Jackson sharks rest in the caverns. Photographers rate the site highly: the combination of vertical relief, colour and fish is rare along the gulf coast. The deeper areas can be silty, so good buoyancy and careful finning keep the visibility up for everyone.

The Drop-off suits Advanced divers and experienced Open Water divers comfortable at around 20 m on a boat dive. It is often paired with the Aldinga Pinnacles, closer inshore, for a two-dive day. Heed local advice about snapper season: when the snapper are running, white sharks occasionally follow the schools, and many local divers avoid the offshore reef at that time.

Boat access. Most divers launch at O'Sullivan Beach boat ramp and run south about 15 km along the coast (roughly 25-40 minutes depending on the boat); small boats can also be launched from the beach at southern Aldinga Beach where vehicle beach access is permitted. The reef lies inside the Aldinga Reef Sanctuary Zone off Snapper Point, about 1-1.5 km offshore.

There are no public moorings; anchor in sand clear of the reef and leave an attendant aboard with a dive flag.

Aldinga faces west into Gulf St Vincent. The best days follow several days of offshore easterly winds, when the sea is flat and visibility can exceed 15 m; westerly and south-westerly winds and swell quickly make it rough and stir the silt in the deeper areas. The summer sea breeze from the south-west usually arrives late morning.

Local guides mention strong currents at times from upwelling along the drop-off, so neap dodge-tide days are easiest. Autumn (March-May) is often the most reliable period. Local advice is not to dive the offshore reef when snapper are schooling (typically late spring-summer), as white sharks sometimes follow the schools.

Anchor on the sand at the base or on the reef top near the edge using GPS. Descend to the base of the wall at 18-20 m, swim along it with the wall on one shoulder exploring caverns and overhangs, then rise to the reef top at 5-8 m for the return leg and safety stop.

Air or EANx 32 for longer time at the wall base.

Occasional shop and club boat trips (Adelaide Scuba, Underwater Sports, The Dive Shack, Sea Wolves) from O'Sullivan Beach; regularly dived by private boats. No daily scheduled charter.