Despite the global significance of nearshore seagrass beds, little is known of their trophodynamic processes. Herbivory by seagrass fishes is thought to be significant but some species previously suspected to be herbivores may be largely detritivorous. Patterns of grazing on the seagrass Posidonia australis by two abundant monacanthid fishes, Meuschenia freycineti and Meuschenia trachylepis, were determined by calculating the removal of seagrass biomass in three Australian estuaries over 14 months. M. freycineti removed significantly more seagrass biomass than M. trachylepis but seagrass biomass removed by both species varied widely across the three estuaries over time. Median amounts of seagrass removal were generally low for all sites compared with standing stock biomass of P. australis leaves and its epiphytes. Nevertheless, grazing by the two species caused high localised rates of removal (up to 90 mg dry weight of P. australis with epibiota per m2) in winter, the season when seagrass growth is least. Such removal rates are far greater than those reported for any other fish grazer in P. australis. These two fish species directly affect seagrass biomass and potentially alter the trophodynamics of P. australis seagrass beds, especially when grazing intensity is high during the season of lowest seagrass growth.