Mahon Pool is a spectacular and demanding shore dive on the exposed northern arm of Maroubra. Beside the ocean baths a rock platform runs out to a kelp-covered reef top at about 7 m, and roughly 75 m from the entry the reef splits into what McFadyen calls a miniature grand canyon: a main gutter running east-south-east with a wall on its southern side, dropping in a series of ledges about three metres apart. Kelp on the tops gives way to sponge gardens on the ledge faces, with overhangs, swim-throughs under large rocks and steps that eventually reach a sand and boulder bottom at 24 m.

Side gutters branch off the main canyon and the whole system is far more complex than its size suggests. Life is good rather than exceptional – blue groper, mado, Maori wrasse, sergeant baker, yellowtail, snapper and bream over the reef, Port Jackson sharks and wobbegongs under the ledges in winter, common seadragons in the kelp and stingrays and fiddler rays on the sand, with abalone in the shallows – but the topography is the reason to come. The catch is access.

The exit is through one of two slits in the rock wall and only the northern one works, only at low tide, and only in a dead-flat sea; at high tide the platform is awash and there is nowhere to get out. The swim is about 300 m each way. McFadyen managed about fifteen dives here over many years, nearly all in winter westerlies, and describes it as only for the experienced advanced diver.

Boat divers can avoid the exit problem entirely by anchoring off the canyon in 23 m, and that is how most people should see it.

Park in the Mahon Pool car park off Marine Parade, North Maroubra (free, fills on sunny weekends). Gear up at the car and walk 100 m down to the rock platform; enter from the small point to the left (north) of the pool and snorkel about 120 m out to the reef edge before descending. Exit through the northern of two slits in the rock wall, which only works at or near low tide, or via a 1.5 m wide sloping rock part-way along.

Boat divers can anchor off the platform where the sounder rises from 26–27 m to 23 m at the boulder line (use the light pole above the pool as a mark).

Requires absolutely millpond seas – a completely calm ocean with a westerly wind – because the exit is a narrow slit in a wave-swept platform. Only dive at or near low tide: at high tide the water washes over the whole rock platform and the exit disappears. Any south or east swell, even 0.5–1 m, rules it out.

That combination happens mostly in winter, which is when nearly all successful dives here are done. Visibility is usually 15–18 m or better. Plan to turn with 60 % of your gas because the return is a 300 m surface/underwater swim of about 20 minutes against whatever the sea is doing.

Snorkel 120 m out, descend onto the 7 m kelp reef and find the main gutter about 75 m from the entry running east-south-east. Follow it down through ledges at roughly 3 m intervals, under the overhangs and swim-throughs, to the sand and boulders at 24 m. Turn no later than 30 minutes and 60 % gas, retrace the gutter shallower, and surface near the reef edge to swim in for the low-tide slit exit.

No regular operator; Abyss and Pro Dive Coogee occasionally guide experienced groups on flat days. Mostly experienced independent divers and St George Scuba Club.