Marys Reef is a relatively recent find: St George Scuba Club member Phil Short discovered it around 2012, and it has become one of the club's favourite Royal National Park sites. It lies south of Barrens Hut and The Balcony. The reef has a flat top at 16–17 m at the mark.

To the south the depth increases to just over 20 m where the sand begins, and the wall there is a sheer drop of over three metres. Towards the north-east of the reef the sand reaches about 24 m. Along the wall are multiple overhangs – the main one is at about 20 m – caves, boulders and gutters, and shallower gullies lead back across the reef.

The wall is lined with sea tulips and sponges, with a few attractive gorgonians. Fish life includes one-spot pullers, eastern blue devilfish, leatherjackets, seapike and large bullrays, and Port Jackson sharks are common from August to October. Nudibranchs such as the splendid chromodoris add colour.

Bottom time is about 30–35 minutes on air or 40–45 minutes on 32% nitrox. Like every site south of Botany Bay, there is no charter boat that dives here, so it requires a private boat or club membership. Anchoring depends on the wind – on sand in a southerly, otherwise on the reef top with plenty of line.

It suits Advanced divers looking for a varied reef with good invertebrate life, and it combines well with a shallower second dive at The Balcony or Marley Point on the same calm-day trip down the national park coast.

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. Turn right and head down past Barrens Hut and The Balcony – about 15 minutes.

McFadyen's GPS is 34° 06.4906' S 151° 09.4142' E, on the reef top at 16–17 m. In a southerly anchor on the sand; otherwise on the reef top, using plenty of rope because the anchor can slip.

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. Port Jackson sharks gather in large numbers in late winter (August–September). Being a little further offshore and deeper, it tolerates slightly more swell than the inshore sites.

Descend south over the wall, then go east along it past the main overhang at about 20 m, through the boulders and gutters, and return via shallower gullies before following the wall back to the anchor. About 30–35 minutes on air or 40–45 minutes on EAN32.

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