Margaret Brock Reef is a broad, shallow limestone reef rising from a seabed shallower than 10 m about 7 km west of Cape Jaffa, with parts that dry at low tide. It is named after the barque Margaret Brock, wrecked there in 1852, and it has claimed four other vessels: the brigantine Maria (1840), the schooner Agnes (1865) and the fishing boats Thunderbird (1964) and Explorer (1977). The Cape Jaffa lighthouse operated on the reef from 1872 until 1973; its lantern room and keepers' quarters were moved to Kingston SE in 1975, and an automatic beacon now stands on the old platform.

The Marine Life Network describes reef for a couple of kilometres in every direction, with ledges, gorgonians and sponges at 8-12 m and both weedy and leafy seadragons, at a point where warm and cool temperate currents meet – which brings some unusual fish. A rock lobster sanctuary, proclaimed in 1973 and re-established under the Fisheries Management Act 2007, prohibits fishing within 1 km of the navigation aid, so the reef around the beacon is a good place to see large lobster and undisturbed reef fish.

Scattered wreckage from the historic wrecks may still be found in the reef, but none is a coherent dive in itself, and all historic remains are protected. The reef's size, shallow depth and remoteness make it a boat-only site for calm days. It suits self-sufficient boat divers wanting seadragons, big lobster and a slice of lighthouse and shipwreck history. Bring cylinders from Mount Gambier or Adelaide.

Cape Jaffa is 20 km south-west of Kingston SE (about 3.5 hours from Adelaide). Launch from the Cape Jaffa marina/ramp or beach-launch as locals do, and run about 7 km west to the reef around the automatic beacon on the old lighthouse platform. The reef extends for a couple of kilometres in every direction, so choose a sheltered side.

Private boats only; watch for cray boats working the area.

The Limestone Coast faces the full Southern Ocean, and prevailing south-westerly swells of 2-4 m are the norm, especially May-October. Open-coast dives need a genuine calm spell: swell under about 1 m, light winds, ideally northerly or north-easterly offshore breezes, and a few settled days for visibility to recover. Summer south-easterly sea breezes build most afternoons, so dive early.

Heavy winter rain brings tannin-stained run-off from drains and creeks that reduces visibility near shore. Autumn (March-May) usually has the most calm days and the best visibility. Parts of the reef dry at low tide and break in any swell; approach the beacon carefully and dive the lee side.

Anchor in sand beside the reef at 8-10 m on the sheltered side, explore the ledges, gorgonians and sponges, search the weed for seadragons, and return to the anchor. Stay outside the breaking shallows.

Rock lobster sanctuary within 1 km radius of the navigation aid – no taking. Historic wreck remains protected.

No regular operator – private boats from Cape Jaffa or Kingston SE.