Upolu Cay is a small sand cay on Upolu Reef, about 31 km north-east of Cairns, and Passions of Paradise notes it is the closest reef to the city on its roster. Cairns Dive Adventures describes the reef as a small, protected lagoon-shaped reef with an inshore classification, scattered bommies and shallow sloping walls, ideal for introductory diving, and lists giant clams, sea cucumbers, nudibranchs, cleaner wrasse, giant trevally, Maori wrasse and hawksbill turtles, along with named sites Jaws, Coral Gardens, ExtaSea, Reef Daytripper, Wonderwall and The Cay. At Passions' ExtaSea mooring, a swing mooring sits in a shallow sandy lagoon about 5 m deep with a coral fringe, and an 18 m drop lies just behind the boat; the operator highlights many soft coral varieties and notes that the lagoon is a popular destination for sailing boats.

Ocean Freedom, which can land guests on the cay in suitable conditions, likens snorkelling there to swimming in an aquarium, with turtles, blue-spotted stingrays, colourful reef fish, sea stars, giant batfish, coral trout, chromis and giant clams in shallow, sheltered water. Upolu is part of a national park and a Marine National Park (Green) Zone. Its closeness to Cairns means a shorter boat ride and more time in the water, but also lower visibility after rain than on the outer reefs.

It suits snorkellers, families and new divers best. Wonder Wall, on the outer edge of the reef, has its own entry. For non-divers in a group, a landing on the cay itself, when conditions permit, is a highlight of the day.

Boat access only. Cairns day boats leave from the Reef Fleet Terminal (1 Spence Street) and the neighbouring Marlin Marina on the city waterfront, with check-in between about 7:00 and 8:00 am; most operators run transfers from city and northern-beaches accommodation, and there is paid parking near the terminal. The run of about 31 km out through Trinity Inlet and across the lagoon to Upolu Reef takes around 75 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Ocean Freedom (fast cruiser), Passions of Paradise (ExtaSea mooring) and Reef Daytripper use Upolu. Ocean Freedom lands guests on the cay when sea conditions allow. Enter from the stern steps or from the cay beach.

The prevailing south-east trade winds (typically 15-25 knots from April to September) are what limit Cairns outer-reef diving: above about 25 knots trips may be cancelled, and exposed eastern moorings are used only in light winds. Summer northerlies are gentler but make north-facing moorings choppy. Box jellyfish and Irukandji are present from about November to May (stinger suits on every dive), and the cyclone season runs November to April, when a system in the Coral Sea can close the reef for days.

Visibility drops after heavy rain and strong winds; spring is usually calmest and clearest. Being closer to the coast, visibility is lower than on the outer reefs, especially after rain. Upolu's lagoon is sheltered and diveable on most days; landing on the cay depends on tide and sea conditions.

Being close to the coast, visibility drops noticeably after heavy rain.

From the ExtaSea swing mooring, descend in the 5 m sandy lagoon, fin to the coral fringe and follow the drop behind the boat to about 18 m, turning at a third of your gas. Return through the shallow fringe and lagoon for a long safety stop.

Ocean Freedom – daily day trips (cay and Wonder Wall); Passions of Paradise – ExtaSea mooring; Reef Daytripper – day sailing trips