Papua New Guinea stretches from just south of the Equator to the Torres Strait, which separates New Guinea from Cape York Peninsula to the south, the northernmost extension of Australia. Mainland Papua New Guinea reaches its maximum north-south expanse of some 510 miles (820 km) along its western border with Indonesian Papua.Almost completely straight, the boundary is formed primarily by the line of longitude 141° E and curves only briefly westward to follow the Fly River for approximately 50 miles (80 km), starting just southwest of Kiunga.

From the western border the land tapers—with a substantial indentation in the south coast formed by the Gulf of Papua—to a fingerlike shape that points southeast toward the  D’Entrecasteaux Islands  and the  Louisiade Archipelago . Off the mainland are a number of small islands and island groups scattered to the north and east and, farther northeast, Bougainville Island and the Bismarck Archipelago; the latter forms a crescent that arcs from the Admiralty Islands in the north to New Britain and Umboi Island, off the mainland’s Huon Peninsula