In New South Wales (NSW), Australia, recent legislative action has focused on identifying key threats to the marine estate in the interest of all stakeholders. The extent of effects estuarine outflows have on NSW marine ecosystems is a knowledge gap that needs to be assessed. We used a systematic literature review to evaluate the knowledge status of the effects of estuaries on marine ecosystems, within the environmental, hydrological, and physicochemical context of NSW. Results focussed on studies that explicitly observed impacts on marine biota (n=59). Consistent with global literature, trace elements and organochlorines were the most frequently studied contaminant types, with reported biological concentrations often below guideline values but commonly detected at the highest concentrations adjacent to urban sources. However, there were studies (n=3) that assessed a wider suite of contaminants – including emerging contaminants such as PFAS – and their effects on the health of marine apex predators. Studies were concentrated in metropolitan areas, thus data was lacking from regions potentially impacted by agriculture. Our review highlights key biological and geographical data gaps in estuarine outflow research in NSW, particularly for the role of estuaries in exporting carbon and nutrients to the oligotrophic waters of NSW.