The Twelve Apostles Marine National Park protects 7,500 hectares of Southern Ocean along the most dramatic 17 km of the Great Ocean Road, from Port Campbell east past the Apostles toward Princetown. It is the second largest marine national park in Victoria and, according to Parks Victoria, holds the state's greatest diversity of invertebrates on its limestone reefs. Underwater the coast continues what the cliffs show above: soft limestone cut by the sea into arches, canyons, overhangs and caves, interspersed with sand plains, with giant kelp forming mini-forests that reach the surface from 10 m in places and extensive beds of common and bull kelp elsewhere. Sponge gardens and colourful invertebrate communities cover the shaded faces, and the fish life includes magpie perch, blue-throat wrasse, Port Jackson sharks, southern rock lobster in large numbers and weedy seadragons on the sand edges. The Loch Ard wreck under Mutton Bird Island lies within the park and is recorded separately.

The named dive that locals talk about is Thunder Cave, a sea cave in the cliffs a short run east of Port Campbell whose underwater entrance is about 25 m deep and, in the words of one regional guide, full of crayfish. Nearby The Blowhole and the collapsed arches and stacks between it and Loch Ard Gorge give wall and cavern diving in 10–25 m, and further east the reefs off the Twelve Apostles themselves are kelp-covered limestone ridges and canyons in 15–30 m that almost nobody has dived because getting there requires a rare calm day and a long run from a crane-launched boat. All of this is expedition-grade diving on a coast where the sea is pounding the cliffs most days of the year. Parks Victoria's guidance is that diving here should only be undertaken by experienced divers with extensive local knowledge, and that is the right frame: this is not a site to plan a holiday around, but for a well-organised group with local boat support on an autumn high, it is some of the wildest and least-seen reef diving in Australia, in a no-take park where the crayfish have never been touched.

Boat only. Small boats are launched by the crane on Port Campbell jetty (licensed permit holders only; Parks Victoria 13 1963) or taken out by local charter operators. Thunder Cave and The Blowhole are 1–3 km east of the inlet, 5–10 minutes' run; the reefs off the Twelve Apostles are 10–12 km east, 25–35 minutes.

Care is needed crossing the wave break east of the Port Campbell inlet entrance. There is no shore access to any of these sites – the cliffs are 40–70 m high – and Gibson Steps (86 stairs to the beach below the Apostles) is a surf beach unsuitable for diving. Warrnambool is the nearest dive shop.

This is the most exposed section of the Victorian coast, taking the full Southern Ocean swell onto vertical limestone cliffs, and Parks Victoria notes that conditions often prevent visits entirely. Boat diving needs swell under about 1 m and light northerly winds; backwash from the cliffs makes it uncomfortable well before that. Southerly and westerly winds and any swell over 1.5 m close everything, including the Port Campbell launch.

Mornings are better than afternoons, autumn better than any other season, and a run of several calm days is needed for the visibility to clear. Cold upwelled water can arrive in summer. Perhaps 10–20 diveable days a year.

At Thunder Cave, drop from the boat off the cave mouth and descend to the floor at about 20–25 m, work into the entrance of the cavern with a torch as far as the surge allows (it is full of crayfish), then explore the walls and ledges outside to either side. Turn at a third of your gas, ascend the wall to a safety stop in the lee and surface with an SMB for pick-up clear of the cliffs. On the outer Apostles reefs, anchor on sand and work the kelp-covered limestone ridges and canyons in 15–25 m.

Thunder Cave is a cavern penetration in surge – torch, reel and cavern experience recommended; stay within the light zone.

No regular charter. Port Campbell Boat Charters and local fishing charters take divers occasionally in calm weather; Warrnambool Diving and Melbourne clubs organise rare trips. Parks Victoria states diving should only be undertaken by experienced divers with extensive local knowledge.