Mackay Reef, with its white-sand coral cay, is one of the two unspoilt reefs Sailaway visits on its luxury snorkel days from Port Douglas. Sailaway describes pristine coral reef with turtles, an amazing display of giant clam gardens, beautiful corals and a wide variety of fish, plus the chance to explore the cay itself. Barrier Reef Australia's listing adds green and hawksbill turtles, often quite large, and seasonal whales, dolphins and reef sharks.

The cay is a low patch of white sand surrounded by turquoise water, and the reef around it is shallow and sheltered enough for snorkellers of all ages. Giant clams are a highlight – a garden of them, their mantles in blues, greens and browns – along with hard and soft corals and reef fish. Turtles are frequent visitors, feeding on the reef and surfacing near snorkellers.

Sailaway's day includes guided snorkel tours with a marine biologist, glass-bottom boat viewing and time on the cay, with about 4.5 hours at two reef locations. Group sizes are limited, and the operator emphasises exclusive access to the cays of Mackay and Undine. WindSwell Adventures also uses Undine and Mackay cays for kitesurfing, describing them as one of Far North Queensland's best-kept secrets, which gives an idea of how wind-exposed the cays can be in the trade-wind season.

Visibility is best in the dry season (May–October). Depths are shallow and not published. Mackay Reef suits snorkellers and families wanting a quieter, more exclusive reef experience than the big pontoon trips, with the bonus of a sand cay.

Two options: from Port Douglas, Sailaway's luxury catamaran departs Crystalbrook Superyacht Marina, Wharf Street (check-in 7:30 am, depart 8:00 am, back about 5:00 pm) and spends about 4.5 hours at Mackay or Undine coral cays (transit time not published); Cape Tribulation-based snorkel operators also visit the Mackay/Undine reefs. Cape Tribulation is about 1.5 hours' drive north of Port Douglas via Mossman and the Daintree River cable ferry. Snorkel entry is from the boat or from the sand cay.

Mackay and Undine are mid-shelf reefs off Cape Tribulation. Their coral cays give some shelter, but the reefs are exposed to wind chop in the south-east trades (April–September) and trips are cancelled in strong winds (roughly 20–25 knots+); beach launches from Cape Tribulation also depend on surf at the beach. May–October gives the calmest seas and clearest water; the wet season (December–April) brings heavy rain, Daintree River plumes that reduce visibility, stinger season (November–May) and cyclone risk.

Snorkel on the higher part of the tide for more water over the reef.

Snorkel from the boat or the cay with the marine-biologist guide over the giant clam gardens and coral, looking for turtles, and stay within the guided area.

Sailaway – luxury catamaran (Sailaway VII) snorkel days from Port Douglas visiting Mackay or Undine (max ~45–52 guests, marine-biologist guides); Cape Tribulation snorkel operators also visit. Scuba not routinely offered.