Lihou Reef is the largest atoll in the Coral Sea, a U-shaped reef enclosing a lagoon roughly 100 km long and 30 km wide, with about 18 small islets on its rim. Marinelife.org.au notes that it was a nature reserve, that marine seaweed beds are prevalent, and that commercial tours were limited to 10 a year. It is now part of the Coral Sea Marine Park, zoned partly National Park Zone and partly Habitat Protection Zone, and part of the Coringa-Herald and Lihou Reefs and Cays Ramsar Site. The Parks Australia zoning map names features including Hermit Crab Islet, East, Central and West Diamond islets, Turtle Islet and South West Cay.

The atoll has a long history of shipwrecks.

The reef itself is almost untouched by divers. Expect shallow lagoon bommies, extensive reef flats and seagrass or seaweed beds inside, and outer walls dropping into deep water outside, with reef sharks, groupers, trevally, turtles and seabirds. Visibility on the outer walls is usually 30 m or more, while the lagoon can be less clear.

Lihou is for self-sufficient expedition divers on a capable vessel with an experienced skipper. Narrow passages into the lagoon are hazardous. The rewards are isolation and a reef system seen by very few people.

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. From Townsville the passage is roughly 550–600 km, two or more days each way.

Narrow passages into the lagoon are hazardous and poorly charted; enter only in good light with experienced local knowledge. 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.

Choose sheltered lagoon-side sites and bommies for most dives, with outer-wall dives only in calm weather using a tender for pick-up. Stay shallow and conservative given the distance from medical help.

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