Hybridization and introgression between species is remarkably common, even between distantly-related taxa. This suggests that the frequency of hybridization between species has been greatly constrained, not by evolutionary divergence in isolation, but by lack of opportunity. This constraint is being relaxed by human-mediated dispersal. Here we document a case where such dispersal of a widespread species of freshwater turtle (Emydura macquarii) into the highly restricted range of a critically endangered endemic turtle (Myuchelys georgesi) has provided opportunity for the two to hybridize. This has raised concerns about the potential impact of hybridization on the endemic species, and its continued persistence in the face of challenges brought about by habitat alteration, increased competition, disease, and genetic pollution. This study serves to highlight the risks associated with human-mediated dispersal, which can bring into contact species that would otherwise have never or rarely met, and so provide opportunity for hybridization and introgression between even distantly-related species with uncertain consequences for already threatened species.