The native Australian crayfish Cherax quadricarinatus (or Redclaw) is a large, highly fecund and fast growing species that has been widely translocated both interstate and internationally. This paper describes a translocated population of the species in Lake Ainsworth, an acidic coastal window lake in New South Wales. This population now represents the southern-most extent of its translocated range within Australia, and coincides with two native crayfishes, Cherax cuspidatus and a highly imperilled species of Tenuibranchiurus. Consistently high trap catches and collection of berried females indicates that the population in Lake Ainsworth is large, well established and self-sustaining. The probable pathway of introduction to the lake was by human action (deliberate or accidental), rather than natural dispersal from previous sites of introduction. Native crayfish were not captured in the lake, but their presence was confirmed in ephemerally connected habitats〜2-3 km away. There are serious concerns about the further spread of the highly invasive C. quadricarinatus and its potential to displace native species.