The Houtman Abrolhos are a unique group of islands, reefs and lagoons located near the Australian continental shelf-edge, some 60 km off the mid-west coast of Western Australia. They are the most southerly true coral reefs to be found in the Indian Ocean. In terms of biological diversity, the islands support a rare combination of both sub-tropical and temperate species located in close proximity.
The group consists of at least 122 small islands and islets that sit atop the three carbonate platforms that comprise the Houtman Abrolhos. These are known as the Easter, Pelsart and Wallabi Groups. The groups spread northwest to southeast across 100 km of ocean and are separated from the mainland by the Geelvink Channel. The outlying North Island is considered as part of the Wallabi Group. They consist of a mix of ancient reefs, sedimentary rocks, living reefs, lagoons, coral debris and sand that combine to form their unusual appearance. Each of the carbonate platforms rises 40 m or so from the shelf below. In general terms, each consists of a windward reef, leeward reef, and lagoon with a central platform. The islands — located on the central platforms or on the leeward reefs — are mainly rocky (composed of limestone or coral rubble) with scant vegetation cover and rise only 3 to 5 m above sea level.
Located at the northern end of the group, 9 km northwest across the Middle Channel from the Easter Group and 58 km off the Australian mainland, is the Wallabi Group, consisting of North Island, East Wallabi Island, West Wallabi Island, Long Island, Beacon Island and a number of smaller islets and emergent rock outcrops. The entire formation upon which these islets and reefs reside measures 17 km in length (northwest to southeast) and up to 10 km in width. The separate formation of North Island lies in an isolated position a farther 14 km to the northwest. The Easter Group, measuring 20 km by 12 km, lies at the centre of the chain and contains the largest island of the Houtman Abrolhos — Rat Island. Other significant landmasses include: Wooded Island, Morley Island, Suomi Island and Alexander Island. Situated 8 km southeast of the Easter Group, across the Zeewyk Channel, are the islands and reefs of the Pelsart Group — the southernmost true coral reefs of the Indian Ocean. Its main islands include Middle Islet, the Mangrove Islets, Square Islet and the elongated Pelsart Islet that stretches for 9 km along the eastern edge of the reef.
The islands lie in the path of the warm Leeuwin Current which flows southwards along the Western Australian coastline from April to October and plays a vital role in modifying both the local climate and in the flora and fauna of the region. Flowing from Indonesian waters, the Leeuwin Current brings warm, low nutrient, tropical waters to the Houtman Abrolhos that helps to maintain water temperatures of 20-22 °C in the winter, thus allowing many species to survive in an area that would normally be outside their range.