The Lapita expansion took Austronesian seafaring peoples with distinctive pottery eastward from the Bismarck Archipelago to western Polynesia during the late second millennium BC, marking the first stage in the settlement of Oceania. Here it is shown that a parallel process also carried Lapita pottery and people many hundreds of kilometres westward along the southern shore of Papua New Guinea. The key site is Hopo, now 4.5km inland owing to the progradation of coastal sand dunes, but originally on the sea edge. Pottery and radiocarbon dates indicate Lapita settlement in this location c.600 BC, and suggest that the long-distance maritime networks linking the entire southern coast of Papua New Guinea in historical times may trace their origin to this period.