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Breeding success of the endemic black-fronted tern (Chlidonias albostriatus) and the identity of predators was studied in the upper Clarence and Acheron Rivers (Molesworth Recreation Reserve), South Island, New Zealand in the 2012, 2013 and 2014 breeding seasons. The catchment supports a nationally and therefore internationally significant population of over 720 breeding birds. A combined total of 1,510 nests were monitored over 3 seasons. Breeding success was poor with only 42.7% of nests hatching at least 1 egg, and average productivity of only 0.13 chicks fledged/nest. Breeding success varied between years and rivers, primarily due to differing predation rates. Predation and nest abandonment following nocturnal predator disturbance were the primary causes of nest failure. A total of 110 filmed predation events at nests identified ferret (Mustela furo), feral cat (Felis catus) and hedgehog (Erinaceus europaeus) as the main predators. Following a beech and tussock masting event, predation by ship rats (Rattus rattus) was significant in 2014. This is the first time predator increases following mast seeding has been shown to impact braided river birds. In contrast, avian predation was low and varied across rivers. Productivity was higher in large colonies (>25 nests) than small colonies, and in early colonies (colony formation before 31 October). A management programme to improve black-fronted tern productivity has been initiated given the national importance of this population, the ease of access to these colonies, and the identification of the invasive predators responsible for current levels of poor breeding success.