The past and present distribution of the blue duck, Hymenolaimus malacorhynchos, in the South Island, is briefly reviewed. The data presented here indicate that the blue duck is widely distributed throughout the mountainous areas of the Island. It is suggested that since early European times there has been a gradual disappearance of ducks from the lower reaches and gorges of many rivers, especially in more developed agricultural areas. Several possible causes of this decline are discussed. Man appears to have been the cause of the restriction in distribution by changing physical parameters of the environment, and by predation.