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Southern New Guinea leads the continental Australian plate into convergence to form the New Guinea Highlands. Extensive inter-tidal deltas are typical of much of this low gradient equatorial coastline. In two adjacent estuaries in Irian Jaya (West Papua), this study uses present elevations of mangrove species zones as a finite indicator of Holocene sea levels from pollen diagrams of estuarine cores. All cores showed mangroves at levels well below the present tidal range, with landward species zones being replaced over time by seaward zones. Results show tectonic subsidence in the recent period, with Late Holocene relative sea-level rise of 0.67 mm year⁻¹.