Floodplain grasslands are highly productive ecosystems that are becoming increasingly degraded and fragmented because of changes to natural flood regimes. In southern Australia, one of the largest floodplain grasslands occurs at Barmah Forest, Victoria. These grasslands are dominated by Pseudoraphis spinescens (Moira grass), an ecologically important mat-forming grass. Although P. spinescens was once widespread in treeless areas within the forest, there has been a dramatic decline in its extent, particularly since the Millennium Drought (2001–09) and subsequent unseasonal and prolonged La Niña-associated flooding from late 2010 to 2012. In this project we mapped and quantified the distribution of P. spinescens to determine its current extent, and to investigate the response of the grasslands to environmental flows. Our detailed on-ground surveys found only 182 hectares of P. spinescens grassland remaining in treeless areas of Barmah Forest, representing ~12% of the total area of treeless plains and lakes. Critically, just 51 hectares of the thick monospecific P. spinescens swards that were a historically important part of the floodplain were located and mapped. Although there was a small increase in the area of P. spinescens in some treeless areas following a favourable flood season in 2013–14, the boundaries of most grassland patches remained relatively stable. Potential drivers of the overall decline in P. spinescens across Barmah Forest include the direct and indirect impacts of altered flood regimes, drought, grazing pressure, and seedbank decline. We discuss strategies for managing these floodplain grasslands, including the use of environmental flows to provide more natural flood regimes, and the challenges of grassland restoration in a regulated and highly modified ecosystem.