Marion Reef is a large, remote atoll at the southern end of the north-western group of Coral Sea reefs, more than 400 km from the mainland. Marinelife.org.au describes a large circular atoll with a lagoon over 30 km across, three main eastern reefs (Marion, Long and Wansfell), coral pinnacles throughout, and three sand cays, with depths of 10–60 m. The Parks Australia zoning map labels Paget, Carola and Brodie cays. It notes excellent wall dives with soft and hard corals, gorgonians and sea whips, and lists fish, small sharks, sea snakes and pelagic schools.

The atoll's layout offers variety. Outside, the walls drop steeply into deep water, their faces covered in soft corals, gorgonians and sea whips where currents run. Inside, coral pinnacles rise from the lagoon floor, giving more sheltered diving when the trades blow. The cays provide landmarks and anchorages.

The fauna reflects Marion's remoteness and low fishing pressure. Reef sharks patrol the walls, sea snakes are common, and schools of pelagic fish including trevally, barracuda and tuna move along the drop-offs. Turtles and seabirds use the cays.

Marion is zoned in part as a National Park Zone, with Habitat Protection and Special Purpose (Trawl) zones around it, under the Coral Sea Marine Park Management Plan. No dive operator visits regularly; it is reached by private expedition vessels, typically from Mackay or Townsville. It suits experienced, self-sufficient divers comfortable on deep walls in current, on a vessel equipped for remote operations, and willing to wait out the weather for a calm window on the outer walls.

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 Mackay or Townsville the passage is roughly 400 km, around two days each way, crossing the Great Barrier Reef lagoon and the outer reef.

Anchor inside the atoll in the lee of the eastern reefs near the cays. 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 outer walls in calm conditions for soft corals, gorgonians and pelagics, staying above 30 m, and use the lagoon pinnacles for easier dives. Tender cover and SMBs are essential on the walls.

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