Seasonal wetlands of the Swan Coastal Plain in southwestern Australia often dry in summer, refill during heavy winter rains, and are commonly surrounded by vegetated catchments. However, the linkage between catchment litter turnover and lake P dynamics is poorly understood. A transect from catchment uplands to the dry lakebed of Thomsons Lake was studied with the progression of the wet season. Attention was given to 1) levels of litter-P release at various sites; 2) spatial patterns of soil P along a wet to dry transect; and 3) dynamic changes in soil P availability in response to changing hydrologic conditions. Phosphorus detained in situ by anion exchange membranes and changes in AEM-extractable P (AEM-P) in surface soils indicated that the first heavy rainfall of the wet season generated significant P loads from catchment litter. In contrast, there was little P released in bare soils without litter. Before rain, soil AEM-P in the lakebed and fringing areas was low, but this increased by 3.4–56 times in the wet season. There was a net decrease in AEM-extractable P, total P, and organic C in the upland catchment but an increase in the wetland during the wet season. The results suggest that heavy rains after six months of drying may generate significant P flux from catchment litter. This would be followed by further leaching and an inferred stimulation of microbial activity during the progress of the wet season. The large increase in AEM-P in lake sites appears related to the input of P from catchment sites through hydrologic processes during the rains and an internal process associated with a ‘drying-rewetting’ effect.