Six Fathom Reef, also called Simpsons Reef, takes its name from its shallowest point – about six fathoms, or 11 m. It sits right at the mouth of Port Hacking north of Jibbon Point in Royal National Park, and its great strength is shelter: Jibbon Bombora blocks the southerly swell, so it is generally the fall-back dive when rough seas close the open coast, and its shallow depth and proximity to the ramps make it a handy second dive. The reef rises gradually from a sandy bottom at about 24 m to its 11 m top.

Small walls of one to two metres ring it, with small overhangs and a couple of little swim-throughs. The western side is the most interesting, with a series of steps: the main wall rising about 5 m, a mid-level wall at 17 m and a deeper wall at 21 m, with sand at almost 27 m below. Fish life is abundant: ladder-finned pomfret, bullseyes, yellowtail, seapike and one-spot pullers in huge numbers, with bream, yellowtail kingfish, trevally, blue gropers, red morwong, eastern blue devils, black reef leatherjackets and wobbegongs, and Port Jackson sharks in spring.

Sponge gardens, gorgonians and lace corals are best on the southern and deeper sections, and rarer sightings include pineapplefish and giant boarfish. A circuit of the reef takes about 20 minutes, and crossing the top allows different routes back to the anchor. Tides do not greatly affect the site, but runoff after heavy rain can dirty the water.

It suits Open Water divers looking for a sheltered, fishy dive and experienced divers wanting a bad-weather option that is better than its reputation.

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. The reef is right at the mouth of Port Hacking, north of Jibbon Point – about 5 minutes from the Cronulla or Bundeena side.

McFadyen's GPS is 34° 04.2050' S 151° 10.2858' E; the traditional marks use the Oak Park toilets and Bundeena. The anchor can be slow to bite on the reef.

The main virtue of Six Fathom Reef is that Jibbon Bombora shelters it from southerly swell, so it is generally dived when conditions are too rough elsewhere. It is still exposed to easterly and north-easterly swell. Tides do not really affect it, but after prolonged heavy rain the Port Hacking outflow makes the water dirty.

The Port Hacking entrance bar can break in big southerly swell, so take care leaving the river.

From the anchor follow the reef anti-clockwise – south, then west and finally north – for about 20 minutes, dropping down the stepped western walls (main wall, 17 m and 21 m) if gas allows, then return across the reef top to the anchor.

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