The 15 Metre Reef – Boomerang Reef to some – is a low ridge of rock lying about fifteen metres off the main Bare Island reef on the western side, running roughly west-north-west to east-south-east at 14–16 m. It is the intermediate step between the classic Right dive and the Deep Wall and one of the most productive macro strips in Sydney. The route out follows the tumbledown wall of the main reef from 7 m to a sand edge at about 12 m, then crosses to the 15 Metre Reef and works along it; landmarks include Sea Horse Rock at 12 m with a piece of the old bridge timber wedged in it, the Cave with its gorgonians and resident blue groper, and Sea Horse Corner where the reef turns back to the main system.

The south-eastern end is a low wall with little growth; the rest is small sponge-covered rocks that repay slow searching. Pygmy pipehorses and big-belly seahorses are the targets, along with more than a dozen nudibranch species, pipefish, red Indianfish, dwarf lionfish, eastern blue devilfish under the ledges, estuary catfish and Sydney cardinalfish in the cracks, Port Jackson sharks in spring and, now and then, a grey nurse shark passing along the sand. McFadyen's warning is worth repeating: do not do this dive until you have dived Bare Island Right multiple times or you will probably get lost, because the reefs here run in several directions and the visibility is often under 10 m.

It needs at least 60 minutes of gas, discipline about not lingering deep, and a firm rule of not surfacing above 10 m because of boats. Done properly it is a long, slow, rewarding dive for photographers and critter hunters.

Bare Island is part of Kamay Botany Bay National Park, 20 km from the CBD. Drive down Anzac Parade to the La Perouse loop and park on the edge of Anzac Parade as close to the island's access road as possible (arrive before 8 am on weekends). Gear up at the car, walk 200 m down the access road and across the timber footbridge to the island, then down the stairs to the rock platform.

Three entries: the island's western point (flat seas only), the Pool between the rock formations (protected, better at high tide), or the Carol's Plaque area, usable in the roughest conditions. From the underwater start at 7 m head west along the tumbledown reef wall; the bottom changes from rocks and kelp to a sand edge at about 12 m after 4.5 minutes, and the 15 Metre Reef lies 15 m off the main reef. Do not attempt it until you have dived Bare Island Right several times.

Sheltered and diveable in flat to small swell from any direction; choose the entry by conditions – western point only when flat, Pool at high tide, Carol's Plaque when it is rough. Tide affects the dive little. Average visibility is 7–10 m and varies from 3 to 20 m, dropping to 1–2 m after heavy rain.

Air is the limiting factor: you need at least 60 minutes of gas for the round trip and must not dawdle in the deeper parts. Never surface unless shallower than 10 m or in an emergency – boats.

Descend at 7 m, head west along the tumbledown wall to the sand edge at 12 m, cross to the 15 Metre Reef and work its length examining the small sponge rocks for pygmy pipehorses and nudibranchs, past Sea Horse Rock (12 m, with old bridge timber) and the Cave with its gorgonians, to Sea Horse Corner. Return along the main reef edge to the exit – 45–60 minutes at a moderate pace.

Guided by Abyss, Pro Dive and Sydney Dive Academy as a macro/navigation dive; experienced independents.