Sisters Island is a small, narrow island east of Sisters Beach surrounded by extensive reef systems stretching 200-300 m on each side. The Marine Life Network guide rates it 7 out of 10, praising extraordinary nudibranch populations, diverse anemones and sponges and numerous fish, with bommies and spires on the northern side - 'The Jewel' on the north-eastern side and 'The Magic Castle' nearby - that are hard to find without a sounder. The Leven Scuba Club calls Sisters Bommie a favourite, easy-to-circumnavigate site.

Michael McFadyen's dive on the north-eastern corner found a bommie topping out at about 7 m, a deeper reef at 20 m and a maximum of 24 m, cut by north-south gutters with overhangs and holes. The kelp, 1.5-2 m high, was bigger than in New South Wales. He recorded red morwong, mosaic and six-spined leatherjackets, seapike, wrasse, colourful sponges, small lace corals, large gorgonians, yellow bryozoans and tiny yellow sea spiders, with 20 m visibility and 19 °C water. Local divers typically combine two or three Rocky Cape and Sisters Island sites in a day, choosing the side of the cape that suits the wind and the state of the tide.

The Rocky Cape and Sisters Island area is, in the Leven Scuba Club's words, the best collection of dive sites on the north-west coast. The quartzite of Rocky Cape - some of the oldest rock in Tasmania - forms gutters, ledges, caves and bommies, and the tidal flow along Bass Strait keeps sponges, gorgonians, sea pens and bryozoans well fed. Michael McFadyen, diving here in February 2008, recorded 18-19 °C water and 20-25 m visibility, figures that surprise divers used to the east coast.

Rocky Cape National Park also protects Aboriginal cave sites and coastal heath above the water, and a Parks Pass is required for vehicles. The nearest hyperbaric chamber is the Royal Hobart Hospital, about four hours' drive away, so conservative profiles and oxygen on the boat are essential.

Sisters Beach is a small village in Rocky Cape National Park, off the Bass Highway about 25 minutes west of Wynyard (1 hr 15 min from Devonport). The Sisters Beach boat ramp is the launch point for Sisters Island (about 3-5 km); Boat Harbour ramp further east is an alternative. Ramps are shallow at low tide.

Run about 3 km from the Sisters Beach ramp to the north-eastern corner of the island (McFadyen: 40 54 02.5 S, 145 34 55.4 E). Divers use live drops or a small anchor buoy rather than anchoring on the reef.

The north-west coast faces Bass Strait: north-westerly to north-easterly winds and westerly swell make it rough, while southerly winds blow offshore and give flat seas - light airs and sunny days are relatively common from January to April. Easterly gales are common in the warmer months and close the eastern faces of Rocky Cape and Sisters Island. Tidal streams run along the coast (eastward on the flood), and several boat sites are only comfortable around slack water.

Difficult to access when an easterly is running; the western shore's low quartzite reefs give swell shelter.

Descend onto the bommie top at about 7 m, drop into the north-south gutters with their overhangs to 20 m, circumnavigate the bommie and finish with a safety stop on top (McFadyen: 33 minutes bottom time plus ascent and stop).

No confirmed commercial dive operator on the north-west coast as of 2026 (older guides mention dive services in Wynyard and Devonport - verify). Sites are dived mainly by the Leven Scuba Club (Ulverstone), other north-west club members and visiting divers with their own boats; Tasmanian Divers (Launceston) runs occasional northern trips.