Many freshwater turtle species have spread beyond their natural distributions through human agency, but introduced populations can be difficult to differentiate from natural ones. The occurrence of an Australian freshwater turtle, Emydura macquarii, in the Greater Sydney region, Australia's most populous urban and peri-urban area, has alternatively been assumed to be natural or deduced to be due to anthropogenic introduction. I apply multiple lines of evidence to show that the occurrence of E. macquarii in the Sydney region is not natural and that the species has proliferated and spread there much more than the notoriously invasive red-eared slider turtle, Trachemys scripta elegans. The E. macquarii population of the Sydney region is highly diverse morphologically, with individuals variously resembling E. macquarii from Queensland, the north coast of New South Wales, and the Murray–Darling Basin. Better understanding is needed of the population genetics of E. macquarii in the Sydney region, the interactions among the region's native and introduced turtles, and the impacts of E. macquarii on biota other than turtles.

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