The middle Oligocene collision of the northern margin of the Australian craton with a complex subduction system resulted in emplacement of a thrust mass and formation of a foreland basin that extended from the Coral Sea to the Indian Ocean. The distribution of carbonate-platform facies in southwestern Papua New Guinea reflects the transition from an Eocene passive margin setting to the early stages of foreland basin evolution. The initial basin configuration, with terrigenous sedimentation confined to the proximal foredeep, allowed carbonate deposition in the shallow environment adjacent to the peripheral forebulge. Subsequent southward migration of the basin resulted in a rapid increase in the area and thickness of carbonate-platform deposition. When the proximal foredeep became filled by detritus shed from the emerging orogen, clastic sediments buried the platform and terminated carbonate deposition. The history of the southern Papua New Guinea carbonate platform illustrates the paradox of carbonate deposition within a foreland basin, whereby basin configuration initially encourages thick and extensive carbonate deposition but inevitably leads to terrigenous inundation and the demise of the carbonate platform.