Papua New Guinea is situated between the stable continental mass of Australia and the deep ocean basin of the Pacific. The largest section is the eastern half of the island of New Guinea, which is dominated by a massive central cordillera, or system of mountain ranges, extending from Indonesia's Irian Jaya to East Cape in Papua New Guinea at the termination of the Owen Stanley Range, and including the nation's highest peak, Mt. Wilhelm (4,509 m/14,793 ft). A second mountain chain fringes the north coast and runs parallel to the central cordillera. Active and recently active volcanoes are prominent features of New Guinea landscapes; there are no glaciers or snowfields. In the lowlands are many swamps and floodplains. Important rivers are the Sepik, flowing about 1,130 km (700 mi) to the north coast, and the Fly, which is navigable for 800 km (500 mi) in the southwest.
The smaller islands of Papua New Guinea are also areas of extreme topographic contrast and generally feature mountain ranges rising directly from the sea or from narrow coastal plains. Volcanic landforms dominate the northern part of New Britain and Bougainville, and some of the smaller islands are extremely volcanic. The Bougainville–New Ireland area comprises Bougainville and Buka islands, the Gazelle Peninsula of New Britain, New Ireland, New Hanover, the St. Matthias group, and the Admiralty Islands.