The Sponge Garden, on the north-eastern side of Sisters Island, is rated 7 out of 10 by the Marine Life Network guide. The reef starts near shore and stretches more than 100 m from the island, with jagged bommies covered in red, orange, pink and yellow sponges, lace bryozoans, gorgonians, nudibranchs, tunicates and hydroids over a thick understorey of pink coralline algae. Barber perch, leatherjackets, wrasse and southern hulafish are common; crayfish and abalone are sparse. The guide recommends drift diving with a competent, alert boat operator. 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.

The Sponge Garden is on the north-eastern side of Sisters Island, about 10 minutes from the Sisters Beach ramp. Drift diving with an alert boat operator is recommended.

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.

Current usually runs; plan a drift with boat cover.

Enter up-current near the island and drift along the reef over the sponge-covered bommies at 15-20 m, surfacing on an SMB for pick-up. Turn the dive at half gas (thirds where current or overhead sections are involved), deploy an SMB before surfacing for the live-boat pick-up, and complete a 3-minute safety stop at about 5 m where depth allows.

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.