Upper Cretaceous coal measures on the West Coast of the South Island of New Zealand include examples of thick, clean bituminous coal that accumulated in geographic proximity to active fluvial regimes. Seams range in longest dimension from less than 1 km to a maximum of about 3 km, and commonly exceed 10 m in thickness. Virtually all seams are dominated by vitrinite, which rarely occurs in amounts less than 80 percent by volume. At Greymouth Coalfield, uppermost Cretaceous seams that are both thick and relatively extensive occur on the western side of the basin, within fluvial sequences containing high proportions of fine sediment. Contemporaneous intervals in the axis of the basin lack thick coal and are dominated by channel sandstones. Localization of major channels to zones of most rapid subsidence is inferred to have allowed clean peat to accumulate in marginal areas that were infrequently flooded. Maceral and mineral assemblages, and variations in coal volatile matter values, indicate that peat accumulated in a variety of settings ranging from well-drained ombrogenous bogs to extremely wet mires in which water levels were frequently above the peat surface. At Pike River Coalfield seams occur in fluvial sequences with both high and low proportions of mudstone. Very thick and locally dirty seams that pass laterally into fluvial or fluviolacustrine mudstones are attributed to mires dissected by small, low-energy streams that were confined for long periods of time. Thinner, cleaner, and more extensive seams that occur interbedded with braided stream conglomerates and sandstones are deduced to have accumulated when major channels were temporarily diverted.