Seabirds face a range of threats, both on land and at sea. Consequently, a high proportion of seabirds worldwide, are currently threatened. This paper reviews the principal threatening processes that affect seabirds in New South Wales. Thirteen of the fourteen species of threatened seabirds that breed in NSW do so only on offshore islands. Nesting habitat on many of these islands has been lost or degraded by introduced mammals such as the European Rabbit Oryctolagus cuniculus, and by invasive weeds such as Kikuyu Grass Pennisetum clandestinum. Predation of eggs, chicks and adults by introduced mammalian predators, particularly the Black Rat Rattus rattus, is the most significant threat that NSW seabirds face while on land. Small species—those weighing less than 600 g—are particularly at risk. At sea, the most potentially damaging threats are currently longline fishing and plastic ingestion. Throughout the oceans of the world, tens of thousands of seabirds are accidentally killed on longlines each year. Mitigation measures are effective for surface-feeding species, such as albatrosses and giant petrels, but are less effective for deep-diving species, such as shearwaters. Significant numbers of seabirds (primarily the Flesh-footed Shearwater) are caught by the domestic longline tuna fishery operating off eastern Australia. Plastic ingestion has a range of lethal and sublethal consequences for seabirds, and the Flesh-footed Shearwater appears particularly to be at risk. There is uncertainty regarding the extent and impact of many threats. This, together with the lack of basic ecological information for many species, currently precludes the development of effective conservation strategies for seabirds in NSW. The current out-of-sight-out-of-mind attitude to the problem must change. Research and conservation action are needed urgently, particularly for species like the Flesh-footed Shearwater that are known to be at risk.