Explore the underwater world of Heron Island, in Australia's Great Barrier Reef, brimming with natural wonders.

Swimming 18 metres underwater in the Coral Sea on a night dive, my wedding ring slides off as I brush past a coral dome. Heart racing, I sweep my torch beam across the sea floor and vertical garden of corals and sponges, knowing it must be here, somewhere. And then I spot it, gleaming like the one ring that bound them all. As if at the cinema, I watch my free hand reach for the golden band and am momentarily surprised to see no runes inside it.

Ring returned to its rightful owner, and pulse steadying, I drift after my companions to explore one of the top 10 favourite dive sites of famed French explorer Jacques Cousteau.

Heron Bommie is the signature dive site of Heron Island, 72 kilometres off the coast of Gladstone, in the southern region of the Great Barrier Reef. This cluster of coral heads, where fish of every shape and hue loiter to be spruced up by cleaner wrasse, is also the most popular night dive for Heron guests.

There are fewer fish after dark but the Bommie’s surprises don’t end with ring stealing. Rounding a coral head we startle a leopard-print moray eel, which whips into a hole above a slumbering turtle. And under practically every ledge are vivid yellow coral polyps wafting flapper-like in the current as they night-feed on plankton.

Heron Island has more natural wonders than a coral cay this small has any right to and the show sometimes starts with a humpback sighting on the two-hour launch trip from Gladstone. But for a bird’s-eye view of leviathans and the island, a leafy green dot on opalescent Heron Reef, it is worth splurging on a helicopter transfer.

Within half an hour of landing, my companions and I are snorkelling with turtles, stingrays, a mixed assortment of lolly-striped and spotted tropical fish — more than 900 species have been recorded — and a posse of business-suit-grey barracuda. Between stripping off sea-soaked wetsuits and falling asleep watching moonlight shimmer on the sea, we tuck into delectable prawns, Moreton Bay bugs and crab at Shearwater Restaurant, named after the wedgetailed shearwaters (mutton birds) that nest on Heron in huge numbers.

Add black noddy terns, and the avian population can peak at 100,000 during the seabird-breeding season in January every year. Eastern reef egrets dot the island’s pisonia trees and spiky pandanas all year, and you may even spot a visiting pair of white-bellied sea eagles.

The island is only 1.7 kilometres in circumference, but a circuit can take hours to complete, with stops to read, picnic and swim. For the more active, it’s even longer: elite open-water swimmers and occasional paddlers descend on Heron Island’s colourful marine life in droves for the annual Great Barrier Reef Swim. Activities in the past have included a swim clinic, snorkelling tours and a three-kilometre island circumnavigation or 800-metre return swim to the rusting wreck of HMCS Protector in the harbour channel.

Landlubbers can get their turtle fill, too. In the summer months, you can easily wile away a night on the beach watching turtles nest and hatchlings scamper towards the foam and the great sea turtle lottery — only one in 1000 reaches adulthood. Six of the world’s seven sea turtle species inhabit the waters around Queensland. Greens and loggerheads nest on Heron and those that haul themselves up the sand to lay (30 to 35 a night last season) were born here. Having spent most of the 30-plus years to breeding age grazing distant feeding grounds, these extraordinary animals find their way home to nest.

All sea turtle species are endangered and need human help, says Tim Harvey, director of the Sea Turtle Foundation. With “sea turtles on tap”, Harvey hopes Heron will become the world’s number-one turtle tourism destination.

Only metres from shore on a guided Reef Walk, we lift a feather mouth sea cucumber from the shallows, which promptly shoots defensive sticky white ‘strings’. “It’s the same stuff Spider-Man shoots out his wrist,” says Asha, our guide. As for the giant clams, algal colonisation turns their voluptuous lips vivid purple, blue and green, giving them a very different look from the toothy grins of the black-tipped reef sharks prowling the harbour as we embark on our last-night sunset cruise.

This is the ideal finale to a perfect few days, with the full moon rising in the east moments before the sun slips from a coppery western sky into the Coral Sea. Is that a hiss as it hits the water? No, it is the bubbles rising in my glass as I make a toast to Heron Island.