In New Zealand, the black swan (Cygnus atratus) is the only large grazer of intertidal seagrass (Zostera muelleri). The present study examined spatial and temporal variations in black swan grazing activity in a large temperate estuary to estimate potential site-specific and estuary-wide impacts on seagrass biomass. We monitored diel swan grazing activities, their seasonal abundance, estimated the rate at which swans consumed seagrass and measured changes in the standing stock of seagrass biomass at four sites in Tauranga Harbour. Black swans foraged primarily at high tide (both during the day and night) and were more numerous at sites with larger meadows, particularly during autumn. Grazing created circular devegetated patches (average size ∼0.28 m²) with 92% of shoots, 25% of roots and 99% of rhizomes removed. We measured an average seagrass consumption rate of 394 g dry mass (DM) swan⁻¹ d⁻¹ in line with consumption rates of other Zostera-feeding waterfowl. At sites where grazing was most intense (annual removal of 19–20% of the average seagrass biomass), we observed a substantial decline (43–69%) in plant biomass in the subsequent growing season. These results suggest that black swan grazing could constitute a threat to seagrass under high grazing pressure.