Until now, the evidence for imported obsidian along the south coast of Papua New Guinea has been limited to eleven excavated sites all dating after c. 2,000 cal. BP. Here we present new archaeological evidence for the sourcing and importation of 4,689 obsidian artefacts from 30 excavated sites at Caution Bay. pXRF analysis of a sample of the artefacts revealed that all but one came from a source on West Fergusson Island some 670 km away. During Lapita (here beginning c. 2,950 cal. BP) and post-Lapita times, the proportion of sites with obsidian artefacts was high, and remained so for a thousand years before suddenly ceasing c. 1,900 cal. BP. Technological analyses of obsidian artefacts from Bogi 1 and ABKL—the richest obsidian sites at Caution Bay—indicate intense unipolar and bipolar reduction and the occasional recycling of unipolar flakes into bipolar cores during both Lapita and post-Lapita times. We suggest that this is a result of the importation of obsidian to Caution Bay through down-the-line exchange.