Dunk Island lies 4 km off the Australian east coast, opposite the town of Mission Beach. The island forms part of the Family Islands National Park and is in the larger Great Barrier Reef World Heritage Area. Dunk Island is by far the largest island in the Family Islands National Park, all of which consist of granite rock. All of the islands were part of the mainland before the last sea level rise began 8,000 years ago. Dunk Island covers 970ha, of which 730 ha is national park and the rest is freehold. Its topography varies, with sandy beaches and rocky shores, slightly undulating slopes, foothills and steeper semi-rugged terrain. Mount Kootaloo is the island's highest point, at 271 m above sea level.
There are over 100 species of birds on Dunk Island, including rare and vulnerable seabirds. During the summer months, the island becomes a breeding site for terns and noddies. The lack of predators, along with a plentiful supply of food from the surrounding reef waters, make it an ideal nesting site. Dunk Island is also home to reptiles such as pythons, tree snakes, geckos and skinks. The island's fringing reefs and surrounding waters are home to an array of marine life such as sea turtles, dugongs, corals, fish, shellfish and crabs. Purtaboi Island (the small island directly out from Dunk Island) is closed and inaccessible for guests from October through to April each year due to the crested terns nesting on the island.