Rock Arch is on the south-west wall of the Flinders Reefs. Marinelife.org.au describes interconnected formations on the south-west wall, with caves and crevices full of colourful invertebrates. Cruising the Edge, the blog of a cruising yacht that anchored behind the main sand cay, describes the south-west reef wall as a dramatic vertical wall with small scattered corals in the first 40 m, a ledge of large gorgonian fans at about 40 m, and an extensive coral cave running about 30 m under the wall with a natural light shaft. The depth sounder recorded 152 m off the wall. The two descriptions appear to refer to the same wall, so they are combined here.

The wall is the main feature: sheer, dropping far below recreational depth, and cut by an arch and interconnected formations near the top. Caves and crevices shelter invertebrates in bright colours: sponges, encrusting corals, sea squirts and feather stars. The cave that extends under the wall is lit by a shaft of daylight, a striking sight, but it is an overhead environment and should be entered only with a guide and a torch, if at all.

The gorgonian ledge at around 40 m is at the limit of recreational diving and suitable only for appropriately trained and equipped divers. Cruising the Edge reports tiger sharks, tawny nurse sharks, yellowfin and dogtooth tuna and turtles in the area.

Rock Arch suits advanced divers comfortable on a deep wall. Because it lies close to the main anchorage, it is one of the more accessible Flinders sites for expedition yachts as well as liveaboards.

Liveaboard only, from Townsville. Mike Ball's Spoilsport visits Flinders on its 7-night Yongala & Coral Sea expedition (depart Breakwater Marina, Townsville, Thursday evening, finish in Cairns) and the 10-night Coral Sea Odyssey; the overnight crossing is about 140 nautical miles (260 km), passing Myrmidon Reef on the outer Great Barrier Reef. Private expedition yachts anchor behind the main sand cay in about 30 m over sand.

The south-west wall lies close to the main sand-cay anchorage, so it is dived from the anchored vessel or a tender.

The Flinders Reefs lie about 250 km off the Queensland coast, roughly 140 nautical miles north-east of Townsville, beyond the outer Great Barrier Reef. The crossing is made overnight through open ocean and is uncomfortable in SE trades over about 20 knots (common June–August). The main sand cay gives some shelter from the trades, and the lagoon pinnacles are more protected than the outer walls.

Currents can be strong through the reef gaps and over the mid-reef bommies. September–December usually gives the calmest seas and 40 m-plus visibility; December–April is cyclone season.

Descend the wall to the depth set in the briefing, explore the arch and interconnected formations, and look into the caves and crevices with a torch. Advanced divers may visit the ledge of gorgonians near 40 m. Ascend along the wall and finish near the reef top.

The 40 m gorgonian ledge and the 30 m-long cave are beyond typical recreational scope; deep and cavern training recommended.

Mike Ball Dive Expeditions (Spoilsport) – 7-night Yongala & Coral Sea (Townsville–Cairns) and 10-night Coral Sea Odyssey, a few departures a year; otherwise private expedition yachts only; also visited by private expedition yachts anchoring at the main sand cay.