In the state of New South Wales (NSW), Australia, recent legislative action has focused on identifying key threats to the marine estate. We used a systematic literature review to evaluate the knowledge status of the effects of estuarine outflows on coastal marine ecosystems, within the environmental, hydrological, and physicochemical context of NSW waters. Results focussed on studies that measured outcomes for marine biota (n = 56). Trace elements and organochlorines were the most frequently studied contaminant types, with reported biological concentrations often below guideline values but detected at the highest concentrations adjacent to urban sources. Few studies measured the impacts of legacy and emerging contaminants to animal health, or the flow on effects to marine ecosystems in NSW. Our review highlights key biological and geographical data gaps in estuarine outflow research in NSW, particularly of the impact on ecosystems of exported carbon and nutrients to the oligotrophic waters of NSW.