Existing at the interface of land and sea, in regions of low topographic relief, mangroves are likely to be some of the first ecosystems that undergo spatial modification due to sea-level rise. The mangrove ecosystems of the Gulf of Papua New Guinea are some of the largest and most pristine in the Asia–Pacific region; they have not been subject to clearance for crustacean farming nor suffered from land reclamation projects. This article establishes through analysis of a time series of aerial photography and satellite imagery from the period 1973–2007, that there have been substantial changes in the distribution of mangroves in this region. These changes include the seaward progradation of the Purari Delta and the regression of the Kikori Delta by an average of 43 m year−1 at its most seaward point. While these findings are likely to be continuations of long-term trends, it is probable that they can be explained by a variety of interacting factors including climate change, sea-level rise, subsistence in the northern Gulf of Papua and changes in sediment dynamics.