Osborn Shoal is the best known of the Bate Bay reefs and the site that most Port Hacking divers learn first. It lies directly off Cronulla Beach and is part of a broad reef system that runs across the bay from Kurnell to Bundeena. The shoal is a flat-topped reef averaging about 12 m, covered in kelp and sponges, with sharp drop-offs on its eastern, northern and southern sides falling to 18 m and in places to 20 m or more; the deepest parts reach about 24 m.

The best-known feature is a cave on the eastern side at about 18 m, with the surrounding bottom at 20–21 m. From there the usual route runs past an overhang and an inlet to a crack that leads back over the reef to the south, with serpent eels in the sand patches. To the west a lower wall with broken reef leads to a sponge garden on the reef top.

The shoal has several named sub-sites – Osborn Shoal East, East Drift, West and South West – each covering a different section of the edge and listed separately here. Fish and invertebrate life is varied: eastern blue gropers (some friendly enough to follow divers), red morwong, cuttlefish, crimson-banded wrasse, Maori wrasse, sergeant baker, yellowtail, Port Jackson sharks in winter, weedy sea dragons, common stingarees, octopus and many nudibranchs, with occasional schools of kingfish. Typical visibility is 9–12 m but has exceeded 25 m (March 2001) and reached about 20 m in March 2026.

With no charter boat now operating out of Port Hacking, the shoal is a private-boat and St George Scuba Club dive, just minutes from the ramps. It suits Open Water divers with some boat experience and is excellent for a relaxed second dive or a drift along the eastern edge.

Boat only. Launch in Port Hacking at Tonkin Park/Gunnamatta Bay (Cronulla), Lilli Pilli or Grays Point and run out past Bass and Flinders Point and Oak Park into Bate Bay. Osborn Shoal lies directly off Cronulla Beach, 5–10 minutes from the Port Hacking entrance; head north-east past Oak Park towards Cape Baily Lighthouse.

The traditional marks are the middle of The Kingsway at Cronulla (west) and the Potter Point outfall breather pipes against the dune ridge immediately left of Cape Baily Lighthouse (north). Anchoring on the reef itself holds poorly – drop on the sand edge and let the boat swing onto the reef. McFadyen's GPS for the main site is 34° 03.4883' S 151° 11.2691' E.

Bate Bay faces south-east and is open to swell from the east through to the south; southerly and south-easterly seas over about 1.5 m make the Bate Bay reefs rough and the Port Hacking bar can be dangerous in big swell. North-westerly and westerly winds give the calmest conditions; summer north-east sea breezes chop up the bay in the afternoon. After heavy rain, Port Hacking and the Cronulla stormwater drains discolour the inshore water.

Visibility typically runs 9–12 m, often over 15 m and occasionally 20–25 m on clean incoming water. In north-easterlies approach from the south-west; in southerlies work into the wind; in westerlies approach from the sand side.

Descend the anchor and swim north-east to find the cave on the eastern edge at 18 m, then continue past the overhang and the inlet to the crack that leads across the reef to the south, checking sand patches for serpent eels. Return over the reef top at 12 m (friendly blue gropers). Alternatively go west along the broken wall to the sponge garden on the reef top.

A drift south along the eastern edge also works.

No regular charter – private boat or dive-club (St George Scuba Club) only; no charter boat currently operates out of Port Hacking.