The Victorian Environment Protection Authority has monitored water quality at four long-term monitoring sites in Port Phillip Bay since the mid-1980s. Port Phillip Bay is a large embayment located within the catchment of one of Australia’s largest cities - Melbourne. The bay comprises a central deeper basin adjacent to a shallower western arm. Waters have a relatively long residence time of around 12 months. Since 1985 Melbourne’s population has increased by over 70% with significant urban expansion to the east and west of the city and down along the Mornington Peninsula. Pollution into Port Phillip Bay comes from three principal sources: treated wastewater effluent, catchment inputs delivered by rivers and creeks and stormwater run-off. Treated wastewater loads have remained relatively stable since the 1980s, although technological advances in treatment have resulted in marked reductions in pollutants such as ammonia. Catchment input loads are highly influenced by climatic variability. Through the late 1990s and early 2000s Melbourne’s climate was dominated by the millennium drought. From the mid-1980s Melbourne has experienced historically drier conditions punctuated by wetter La Nina years. Analysis indicates that, although water quality has been strongly influenced by climatic events, it has remained relatively stable over this period.