Two terrestrial Crustacea, Puhuruhuru patersoni (Amphipoda: Talitridae) and Styloniscus otakensis (Isopoda: Styloniscidae), were discovered on sub-Antarctic Macquarie Island in 1991 and 1992, respectively. Each species was identified as having been described, and previously only known, from South Island, New Zealand and from some of New Zealand's southern offshore islands. The distributions of the two species on Macquarie Island have been mapped in detail on four occasions over the last fifteen years, each mapping extending previously existing distributions or showing changes from the earlier records. Details of all four years' mapping are figured. It is concluded that these species were most likely introduced between 1810 and 1919 and that populations of the amphipod have not expanded to any extent in the twelve years between 1992 and 2004 but that the isopod has slightly increased its range. Both species are macrodetritivores, a trophic group not well represented on the island, so there is the possibility of an irreversible change to the Macquarie Island ecosystem if they become more widespread with warming climates. However no such change has yet been observed. The advantages and feasibility of removing these two exotic species from the island is discussed, as are the possible routes by which the species were introduced to the island.