Three Sisters is widely regarded as one of the best dives on the Cairns outer reef; Passions of Paradise calls it one of the truly great dives on the Great Barrier Reef. Three large bommies rise from a sandy floor at depths of roughly 12 to more than 30 m, the second and third sisters reaching to within a metre of the surface. The gap between those two is only about 3 m wide and is known as Gorgonian Gorge for the sea fans that line it; bigger fish use it as a corridor, and swimming through it is the highlight of most dives here.

Each sister has a slightly different character. Great Barrier Reef Tours describes the second as a good place to explore the base at about 19 m, with blue-spotted lagoon rays and sharks on the sand, and the third as home to schools of fusiliers and snapper with brilliant yellow gorgonians along its wall. The bommies are pocked with swim-throughs, overhangs and crevices, and support schools of snapper, surgeonfish and sweetlips on their flanks.

Sleeping whitetip reef sharks, turtles, moray eels, garden eels on the sand and cruising barracuda complete the picture, and the coral cover is healthy and varied. The depth range lets divers of all levels enjoy it: new divers can stay on the upper faces, while experienced divers can drop to the base of the outer bommie. Snorkellers can see the tops of the second and third sisters in clear water.

It is also a strong night dive in calm weather. Note that Michaelmas Reef has an unrelated site of the same name.

Boat access only. Cairns day boats leave from the Reef Fleet Terminal (1 Spence Street) and the neighbouring Marlin Marina on the city waterfront, with check-in between about 7:00 and 8:00 am; most operators run transfers from city and northern-beaches accommodation, and there is paid parking near the terminal. The run of about 55 km out through Trinity Inlet and across the lagoon to Milln Reef takes around 76 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Three Sisters is a Passions of Paradise and Silverswift mooring and is also visited on Pro Dive Cairns liveaboards. Enter from the stern and descend the mooring toward the first bommie.

The prevailing south-east trade winds (typically 15-25 knots from April to September) are what limit Cairns outer-reef diving: above about 25 knots trips may be cancelled, and exposed eastern moorings are used only in light winds. Summer northerlies are gentler but make north-facing moorings choppy. Box jellyfish and Irukandji are present from about November to May (stinger suits on every dive), and the cyclone season runs November to April, when a system in the Coral Sea can close the reef for days.

Visibility drops after heavy rain and strong winds; spring is usually calmest and clearest. Three Sisters is fairly sheltered and usable in most trade-wind conditions; current can funnel through the gorge between the second and third bommies on a running tide.

Descend to the base of the first sister at 15-18 m, cross the sand to the second (around 19 m at its base) and then pass through the Gorgonian Gorge, the 3 m gap between the second and third sisters. Circle the third sister, then ascend and return across the tops at 5-8 m, finishing with a safety stop on the shallowest bommie.

Silverswift – daily day trips (Milln is one of its core reefs); Passions of Paradise – daily sailing-catamaran trips to Milln moorings; Pro Dive Cairns – 3-day/2-night liveaboard including Milln night dives