Spring and summer diets of feral house cats (Felis catus), ferrets (Mustela furo) and stoats (M. erminea) were studied in grassland surrounding breeding areas of yellow-eyed penguins (Megadyptes antipodes), a regionally threatened native species. All three predator species ate large numbers of young rabbits (Oryctolagus cuniculus) and birds. Stoats also relied heavily on mice (Mus musculus). Use of rabbits increased in rank order of increasing predator size, and male stoats ate more lagomorphs than female stoats. Diet differences may reflect character displacement as a result of exploitation competition, but interference competition or predation may force the smaller species to exploit micro-habitats with increased ground cover and consequent increased availability of smaller prey. Reduction of predation of native species like yellow-eyed penguins by decreasing or increasing staple mammal prey numbers of the introduced predators may provide lasting conservation benefits, but could also trigger diet changes that increase risk to endangered wildlife.