Michaelmas Cay is a small, vegetated sand island, about 1.5 hectares in area and rising only about 3.5 m above the sea, on the western tip of Michaelmas Reef. It is a national park and one of the most important seabird breeding sites on the northern Great Barrier Reef: Queensland Parks records up to 20,000 birds of 23 species in summer, dominated by sooty terns and common noddies, and the din and smell of the colony are part of the experience. Visitors may only land between 9:30 am and 3 pm and must stay within a roped area on the beach.

Underwater, the site is a shallow, sheltered coral garden on the cay's protected western side, and Cairns Dive Adventures lists two snorkel areas, the Main Beach and the Outer Edge Wall, with depths of 1.5-12 m, no significant current and visibility of 10-15 m. Its most famous residents are giant clams, some thought to be over 60 years old, lying on the reef flat within an easy swim of the beach. Turtles are common, and the coral supports a good range of reef fish, parrotfish, wrasse, damsels, butterflyfish and anemonefish, with reef sharks sometimes seen on the edge.

Ocean Spirit, which runs a sailing catamaran to the cay, holds advanced ecotourism certification for the trip and offers introductory dives from the beach as well as certified dives. The combination of easy, shallow diving, a bird colony and a sand cay makes it a classic family day. Experienced divers will find it limited in depth and scale, but it is a gentle and memorable introduction to the reef.

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 40 km out through Trinity Inlet and across the lagoon to Michaelmas Cay takes around 90 minutes on a fast catamaran and noticeably longer on slower dive boats or into a south-easterly chop.

Ocean Spirit Cruises runs a sailing catamaran to Michaelmas Cay and is the main operator there; guests are ferried to the cay beach by tender. Cay access is permitted only between 9:30 am and 3 pm, inside the roped visitor area. Introductory dives start from the beach; certified divers dive from the vessel or tender.

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. The western side of the cay is sheltered in the prevailing south-easterlies and is diveable on most days; visibility is typically 10-15 m, lower after rain.

Cay access is restricted to 9:30 am-3 pm, and in the summer breeding season up to 20,000 seabirds nest there.

Introductory divers wade in from the main beach and descend over sand to the coral edge at 3-6 m. Certified divers work along the outer edge wall to about 12 m, visiting the giant clams on the reef flat before returning to shallow water for the safety stop.

Ocean Spirit Cruises – daily sailing-catamaran trips with snorkelling, introductory and certified dives, and guided seabird viewing; Michaelmas is also visited daily by Ocean Spirit Cruises (cay) and by Tusa Reef Tours; the mooring used on any day is chosen by the skipper according to wind, tide and other boats