The Gap is the reef at the foot of the most famous cliff in Sydney, a few hundred metres south of the harbour entrance. From the water the setting is dramatic: sheer sandstone rising 60 m above you, the Dunbar anchor site on the platform below the lookout, and constant ferry and shipping movement at the Heads behind. Underwater the site is a large field of boulders that have calved off the cliff over thousands of years and now lie piled on a rock shelf between about 10 and 23 m.

The reef runs north-west to south-east, meeting clean sand at 23 m. Some of the boulders are the size of a room and where they lean against each other they form swim-throughs, small caves and short sheer walls of three to five metres. Almost every surface is covered in sponges, ascidians, sea squirts and small gorgonians; the sponge colour here is as good as any of the shallow Eastern Suburbs sites because the water is clean and the site is not dived heavily.

Fish life is the point of the dive. Black drummer and luderick school in the surge zone, one-spot pullers and mado hang over the boulders, and black reef and six-spined leatherjackets pick at the growth. Yellowtail kingfish often sweep through in large schools and bream and blue groper are common.

Weedy seadragons are reported occasionally in the kelp at the edge of the sand but are not reliable here. Historically the site matters because it is where the Dunbar struck in August 1857 with the loss of 121 lives; scattered wreckage and anchors lie in the shallows immediately north of the main boulder field and are covered in the separate Dunbar entry. The Gap is best treated as an intermediate boat dive: depths are modest, navigation is simple along the cliff line, and it is one of the few outer-coast sites that works in a southerly.

It suits divers who want a scenic wide-angle dive with real fish life within twenty minutes of a harbour ramp.

Boat only. Charters and private boats leave Sydney Harbour (Rose Bay ramp, Rushcutters Bay or the Spit) and turn south immediately outside South Head; it is only a few hundred metres past the Hornby Light to the lower section of cliffs below The Gap lookout, perhaps 15–25 minutes from Rose Bay. Run in towards the cliff until the sounder shows the sand at about 23 m giving way to rock, and anchor on the edge of the boulder field.

Divers descend the anchor line; there is no shore access – the cliffs are sheer and the platform below is the Dunbar wreck zone.

The reef sits in the lee of South Head and is partly protected from southerly seas, so it is one of the ocean sites that can still be dived when a 2 m southerly swell has blown out Bondi and Coogee. It is exposed to easterly and north-easterly swell, which rebounds off the cliffs and creates surge and washing-machine conditions in the shallow boulders. Avoid strong north-easterlies (typical summer afternoons) and any swell over about 1.5 m from the east.

Current is occasional and can run strongly along the cliff after a period of southerly weather. Morning dives in winter westerlies are best. Shipping and ferry traffic at the Heads is constant – ascend on the anchor line.

Descend the anchor to the sand/rock edge at about 23 m and work north-west along the base of the boulder field, poking into the caves and swim-throughs under the room-sized rocks. At half air turn and come back shallower along the 14–16 m level on top of the boulders, where the kingfish and drummer school. Finish over the 10 m rocks near the cliff for a safety stop on the anchor line.

Dive Centre Bondi, Pro Dive Coogee, Frog Dive and Southern Cross Divers run boat dives here on weekends, usually paired with The Colours or the Dunbar; Abyss and Sydney Dive Academy also visit when southerly swell rules out the Eastern Suburbs shore sites.