Michaelmas and Upolu Cays National Park, Nature reserve in Douglas Shire, Australia.

Michaelmas and Upolu Cays National Park consists of two coral cays on the Great Barrier Reef, 33 km east of Cairns. Michaelmas Cay, on Michaelmas Reef, is a vegetated sand island about 8.5 km from the smaller, unvegetated Upolu Cay on Arlington Reef. Michaelmas Cay is stabilized by beachrock but shifts about one metre a year as sand moves.

In 1926 geologists bored 182.8 m into Michaelmas Cay, hitting reef rock to 115.3 m and quartz sand below. The two cays lie about 8.5 km apart on Michaelmas Reef and the Arlington reef complex. Michaelmas Cay shifts roughly a metre a year as sand moves with wind and current, while Upolu Cay, without beachrock, changes shape faster.

Michaelmas Cay is a globally important nesting site for seabirds: over 20,000 nests have been counted at the peak of the breeding season, and BirdLife International lists it as an Important Bird Area for holding more than 1% of the world populations of greater and lesser crested terns. Sooty terns, common noddies, brown boobies and ruddy turnstones also breed there, and green turtles occasionally lay eggs. Visitors view the colony from a roped-off section of the cay.

Access to the park requires organized boat tours departing from Cairns, with the most favorable conditions occurring from May through October. Bring sun protection and note that conditions at sea can change depending on weather patterns.

Michaelmas Cay functions as a breeding ground for more than 20 seabird species that gather in visible concentrations. The density of bird activity at such a small location creates an unusual natural spectacle.