Substantial marcasite and pyrite were recently identified in the upper-most unoxidised Holocene clay-peat sediments at Bungawalbin Swamp, a coastal backswamp on the Richmond River floodplain, north-east New South Wales, Australia. Marcasite (FeS2), the mineral dimorph of pyrite, is considered a rare secondary mineral in estuarine sediments and its abundance at Bungawalbin Swamp is highly unusual. The morphology and distribution of marcasite in the sulfidic sedimentary profile was examined and compared with the palynological record. Marcasite, recognised by its distinctive platy morphology and disulfide composition, occurred exclusively within organic remnants in only the upper most 1 m of the sulfidic sediment layer. Pyrite occurred throughout the sulfidic profile. A dramatic change in the sedimentary conditions at Bungawalbin Swamp from a marine environment, characterised by the presence of avicenniaceae (i.e. mangrove), to a fresh-brackish environment correlates directly with the occurrence of marcasite. The occurrence of substantial marcasite may provide a valuable environmental proxy of fresh-brackish depositional swamp environments.