There is a paucity of information on the diversity and occurrence of freshwater fish parasites in Australia. This study investigates the distribution and occurrence of a parasitic nematode of the genus Contracaecum in freshwater fish from south-eastern Australia. Fish (n = 508) belonging to nine species and eight families were collected from eight wetlands associated with the Murrumbidgee River floodplain in the southern Murray–Darling Basin and subjected to laboratory examination. Third-stage nematode larvae were found in eight of the nine fish species. The exception was the fly-specked hardeyhead (Craterocephalus stercusmuscarum), although only one specimen of this species was examined. Nematode larvae were identified as Contracaecum bancrofti using a combined morphological and molecular approach. The parasite was most prevalent in weather loach (Misgurnus anguillicaudatus; 34.8%) and Australian smelt (Retropinna semoni; 21.4%), followed by carp (Cyprinus carpio; 9.5%), of which the former and latter are non-native species. Contracaecum prevalence differed between locations, with Two Bridges having the highest number of infected fish. This may be due to the higher number of suitable host species collected at these localities. Contracaecum spp. are parasites with low host specificity that have also been reported in Australian marine fish, humans and piscivorous birds. The high parasite prevalence in the two abundant non-native fish species in the region suggests that they act as a suitable host for this endemic parasite, resulting in the increase in the parasite population. It would be interesting to study host–parasite interactions in this area, especially if introduced fish populations declined dramatically in the attempt to eradicate them or control their population.