The Old Aquarium takes its name from the domed aquarium buildings that once stood at the end of Viaduct Road, and it is the ocean-facing reef immediately south of the Warrnambool breakwater, also known as Breakwater Rock or the Breakwater Reefs. Where the inside of the breakwater is a gentle wall dive, the outside is, in the Scuba Doctor's phrase, very rugged: crevices, overhangs, swim-throughs and kelp forests in 4–14 m, cut into limestone by the swell that pounds this shore most of the year. It is right on the boundary of the Merri Marine Sanctuary and is renowned locally as a place to catch abalone and crayfish, so divers need to know exactly where the sanctuary line runs before taking anything. The residents listed by the Scuba Doctor are longsnout boarfish, Port Jackson sharks, stingrays, southern rock lobster and black-lip abalone; to those add the usual western Victorian community of blue-throat wrasse, magpie perch, sea sweep, herring cale, zebrafish and leatherjackets, cuttlefish in the crevices, and the occasional fur seal from the Lady Julia Percy colony.

Access is easy in distance – the car park is a minute from the water – but the entry over the rocks onto the platform and out through the surge zone is the real test, and it should only be attempted at low tide on a low swell when the platform is not breaking. Given those conditions it is a better dive than the inside of the breakwater: clearer water, real structure, bigger fish and a chance of sharks and seals, all within 100 m of the car. It works well as a night dive because the breakwater lights give a fixed reference for the return. It suits Open Water divers with some surf experience and is the natural step up from the inside wall for anyone spending a few days in Warrnambool.

Follow Viaduct Road to the breakwater and park in the car park near the old Aquarium domes at the seaward end of the road, a minute's walk from the water. Gear up at the car, scramble over the rocks onto the reef platform on the ocean (southern) side of Breakwater Rock and enter from the platform edge, swimming out over the reef. Exit the same way, timing it between sets.

The breakwater lights help orientation on a night dive. Warrnambool Diving on Fairy Street is 2 km away.

The site faces south into the Southern Ocean and the Scuba Doctor warns that seas here can be treacherous and require close monitoring before entry. Dive at low tide with a low swell (under about 0.8 m) and light northerly winds; southerly and westerly winds and swell over a metre make the platform entry and the surge on the reef dangerous. Visibility is generally better than inside the breakwater because the reef is swept clean, and is best after settled northerly weather in autumn.

Check the platform from the car park – if it is white, dive inside the breakwater instead.

Enter from the platform at low tide, descend in 4–5 m and work south-east over the reef through the crevices and swim-throughs, dropping to 10–14 m on the outer edge where the boarfish and Port Jackson sharks sit under the overhangs. Turn at half gas, navigate back north-west toward the breakwater (use the wall or its lights as a reference) and exit at the platform between sets.

Warrnambool Diving (Fairy Street) – air fills, PADI courses, divemaster-led local shore dives on request; Daktari Surf & Dive (Warrnambool) – snorkel gear and guided snorkels; Warrnambool dive club members dive the sanctuary regularly on calm days.