Port MacDonnell calls itself Australia's southern rock lobster capital, and its harbour – protected by a large artificial rock breakwater – shelters the biggest lobster fleet in South Australia. The Marine Life Network lists the breakwater as a sheltered muck dive at 2-5 m with good fish life and possible crayfish, the one reliable ocean dive near Mount Gambier when the Southern Ocean is up.

The dive runs along the inside of the breakwater, where boulders meet sand. The rocks shelter octopus, crabs, small rock lobster, cuttlefish and blennies, while the sand and seagrass patches hold rays, flathead, gurnard and a changing cast of juvenile fish drawn by the harbour's scraps. It is not pretty in the reef sense – visibility is often only a few metres and the bottom can be silty – but patient macro divers find plenty, and the shallow depth means long dives.

The town itself is 28 km from Mount Gambier and the southernmost town in the state; it was proclaimed a port in 1860 and by the 1880s was one of Australia's busiest, shipping wool and wheat despite its exposure and many shipwrecks. Nearby Cape Northumberland has rock formations with lobster and little penguin colonies, and Racecourse Bay to the west offers limestone reef on calm days.

Boat traffic is the main issue: stay out of the channel and ramp area, tow a float, and talk to the harbour users. It suits new divers, macro photographers and cave divers wanting a salty change on a windy day.

Port MacDonnell is 28 km south of Mount Gambier on the Riddoch Highway. Drive to the harbour on the foreshore, park near the boat ramp or breakwater (away from trailer parking), gear up at the car and walk a short distance to enter from the sandy shore or low rocks inside the breakwater. Stay on the harbour side and clear of the boat channel and ramp, where the lobster fleet moves at all hours in season.

The breakwater makes this one of the few Limestone Coast ocean sites diveable in a swell, but big south-westerly seas surge around the breakwater tip and cloud the harbour; strong southerlies push swell into the entrance. Visibility is usually modest (2-8 m) and drops after storms and rain. Boat traffic is heaviest early morning and late afternoon during lobster season (October-May).

Mornings before the sea breeze are calmest.

Enter inside the breakwater, follow the rock wall at 2-5 m looking in the boulders and on the sand edge for octopus, crabs, cuttlefish and small fish, and return along the same line; use a surface marker.

No regular operator. Dive Experience (Mount Gambier, 28 km) for fills, hire and local advice.