Mellish Reef is the most distant Coral Sea reef from the Australian mainland, about 580 nautical miles east of Cairns according to marinelife.org.au. It describes a boomerang-shaped reef about 10 by 3 km, almost completely submerged at high tide, with a lagoon about 12 m deep, a good anchorage and abundant seabirds. Herald's Beacon Islet is the only land.

Few divers have ever been here. The distance means a passage of three days or more each way, so visits are limited to occasional research voyages, long-range expeditions and ocean-cruising yachts. No dive sites are named, and there is no published description of the underwater terrain beyond the lagoon depth.

Based on the general pattern of isolated Coral Sea reefs, the lagoon at around 12 m will offer sheltered diving over coral and sand, while the outer slopes drop steeply into deep water with clear oceanic visibility. Low human impact suggests healthy populations of reef sharks, groupers, trevally and other large fish, and the seabird colonies on the islet point to a productive ecosystem. These expectations are indicative only.

Mellish is suitable only for highly self-sufficient divers on long-range vessels, with a skipper experienced in open-ocean passages and a plan for medical emergencies, since help is several days away. Diving should be kept shallow and conservative. For divers who value remoteness above all, few places in Australian waters compare. The weather window is the critical planning factor: the passage is exposed to the full south-east trades for most of the year.

There is no scheduled dive operator. Access is by private expedition charter, research voyage or cruising yacht with Coral Sea experience, self-sufficient for several days and carrying compressor, oxygen and satellite communications. Mellish Reef is about 580 nautical miles (1,070 km) east of Cairns, three or more days' passage each way – among the most remote diving in Australian waters.

There is a good anchorage in the 12 m lagoon. Commercial tours need Parks Australia authorisation, and some islands are closed to landing.

These are remote oceanic reefs with no shelter from the south-east trades beyond their own reef rims, which blow 15–25 knots for much of April to October and make the multi-day open-ocean passage rough. Expeditions target calm windows, most often September to December; the lagoon anchorages give protection in moderate trades but the outer walls are dived only in light winds. December to April is cyclone season and trips are rarely scheduled then.

Currents on the walls and through passes can be strong; visibility is usually 30 m or more.

Dive the lagoon at around 12 m from the anchorage and, in calm weather, the outer reef slope with tender cover. Keep profiles conservative given the extreme distance from medical help.

No regular operator – occasional private expedition charters, research voyages and cruising yachts only; any commercial tour requires Parks Australia authorisation.