This study documents the helminth assemblages of grassland melomys: Melomys burtoni (Ramsay, 1887), Melomys sp. cf. burtoni and Melomys lutillus (Thomas, 1913). In total, 22 helminth species comprising one cestode and 21 nematodes from 112 hosts were found. All the specimens of Physaloptera spp. examined proved to be Physaloptera banfieldi Johnston & Mawson, 1941 with P. troughtoni Johnston & Mawson, 1941 the junior synonym. The dominant helminth group was the trichostrongyloid nematodes including Odilia melomyos (Mawson, 1961) and O. mackerrasae (Mawson, 1961). The most prevalent, O. melomyos, occurred in each of the host species across all areas sampled (New South Wales, Northern Territory, Papua New Guinea, Queensland and Western Australia). The helminth assemblage of M. burtoni from hosts from New South Wales and Queensland was the most diverse. The helminths of M. burtoni from the Northern Territory and of M. lutillus from Papua New Guinea were subsets of that assemblage. That of M. cf. burtoni from Western Australia, with only six helminth species, was not dominated by trichostrongyloids, three of the six species were not found in other localities, and, with Sorensen’s Indices of 18.2% when compared with the helminths from Papua New Guinea and the Northern Territory and 24% when compared with Queensland, was the least similar. No substantial differences were found between the helminth assemblages of the grassland melomys group, excluding M. cf. burtoni, and Melomys cervinipes (Gould, 1852), the fawn footed melomys. This was reflected in a Sorensen’s Index of 67.9%. The time between the arrival of Melomys into Australia during the Pleistocene and the present day suggests that the trichostrongyloids O. melomyos, O. mackerrasae and O. mawsonae (Durette-Desset, 1969) may have travelled with their rodent hosts from New Guinea to Australia and other helminths in the assemblage may have been acquired in Australia.