Overfishing threatens the viability of high seas living resources. Furthermore, controls to prevent overfishing are inadequate. Illegal, Unreported and Unregulated (IUU) fishing is a product of overfishing and affects all marine fisheries. Southern Ocean fisheries have been particularly targeted by IUU fishing. The efforts of the Convention for the Conservation of Antarctic Marine Living Resources Commission and the Australian government to adopt complementary measures to deter IUU fishing in the Southern Ocean form the basis of this study, although the examination is conducted following a comprehensive review of the legal and other factors contributing to the development of IUU fishing as well as the applicable international law. The author concludes that in the absence of flag State control, alternative measures and strategies have proved to be effective in influencing the behaviour of IUU fishing vessels. By improving coastal State surveillance, enhancing regional cooperation, imposing port and market State controls, establishing IUU vessel databases and vigorously prosecuting offenders, Regional Fisheries Management Organisations and coastal States can jointly increase the risk of conducting IUU fishing activities. At the same time financial returns can be diminished so that engaging in IUU fishing becomes economically unattractive. It is concluded that the application of a combination of these measures is required in order to achieve a degree of success in deterring IUU operators.