Tumbledowns lies just south of Barrens Hut, where the reef edge turns back in towards the coast before swinging south again, and it is named for the tumbled boulders that characterise it. Running in, the sand is at about 24 m, and a short distance from the sand edge the reef comes up to 15 m and then 12 m. Small rocks and boulders are scattered along the sand, and the reef itself has a number of small canyons, a few swim-throughs and huge boulders, with the shallows reaching about 7 m.

The sponge life is very good. The fish and animal list is long: weedy sea dragons, rays and fiddler rays (McFadyen once counted 11 on one dive), and in autumn and winter Port Jackson sharks – as many as 50 on one dive. Schools of yellowtail, ladder-finned pomfret, seapike, silver sweep and one-spot pullers hang over the reef, with luderick, bream, morwong and heaps of leatherjackets among the boulders.

McFadyen has also seen an eagle ray and, in April 2004, a sunfish. Because the reef runs for a long way and any section is diveable, Tumbledowns is flexible: boats can simply run in on the pile of rocks and anchor where the reef rises from 24 m, and in a gentle current it makes a relaxed drift dive. The range of depths lets divers tailor the profile, so it suits Open Water divers as well as experienced divers looking for a varied, fishy dive close to Port Hacking.

Boat only. Launch in Port Hacking (Gunnamatta Bay/Tonkin Park at Cronulla, Lilli Pilli or Bundeena ramps), run out through the entrance and turn right (south), passing between Jibbon Bombora and Jibbon Head in suitable weather, then follow the Royal National Park cliffs south. Tumbledowns is just south of Barrens Hut, about 10 minutes from the entrance.

McFadyen's GPS is 34° 05.5489' S 151° 10.1773' E; run in towards the shore in line with the prominent pile of rocks on the cliff and anchor once the reef comes up from 24 m. Any section of this coastline is suitable – you do not need to be exact. The site is also run as a drift dive along the reef edge.

The Royal National Park coast south of Jibbon faces east and takes swell from the north-east round to the south-east; southerly swell partly wraps around. These sites lie close to the cliffs, and the shallower ones need calm seas so that boats can anchor near shore and divers are not caught in surge – plan on seas under about 1–1.5 m. Westerly and north-westerly winds give the best conditions; a strong north-easter makes the return to Port Hacking rough.

Visibility is generally good because the coast is far from urban runoff, typically 12–20 m, occasionally 30 m. The shallower sections at 7–12 m surge in easterly swell, so keep to the deeper edge unless it is calm. A northerly current occasionally runs along the coast, which suits drift diving.

Descend to the sand at about 24 m, follow the reef edge either way, then come up to the canyons and swim-throughs at 15–17 m and, if gas allows, up into the boulders at around 7 m before returning to the anchor. In a current, drift along the edge with an SMB.

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