Simple SummaryThe New Zealand species of a genus of black fungus gnats show clear phylogeographic patterns, at the species level and above. North Island harbours more species than South Island, and according to our phylogeographic analyses, was more often the starting point of dispersal events to South Island than vice versa. We therefore deduce that North Island is a radiation centre. Initial colonisations of New Zealand took place three times, most likely starting from Australia, with the earliest in the late Miocene.AbstractSciaridae (Diptera) is a widespread insect family of which some species can reach high abundances in arboreal habitats. This trait, together with their (passive) mobility, enables them to quickly colonise suitable habitats. To reveal the biogeographic history of the New Zealand members of the sciarid genusPseudolycoriella, we analysed three molecular markers of selected species and populations in a Bayesian approach. At the intra- and interspecific levels, we detected a pattern of northern richness vs. southern purity, which has probably developed as a result of Pleistocene glacial cycles. Since the late Miocene, we identified 13 dispersal events across the sea strait separating New Zealand’s main islands. As nine of these dispersal events were south-directed, North Island can be considered the centre of radiation for this genus. An unequivocal re-colonisation of North Island was only observed once. Based on the inclusion of three undescribed species from Tasmania and on previously published data, three colonisations of New Zealand are likely, all of them assumed to be of Australian origin. One of these most probably took place during the late Miocene, and the other two during the late Pliocene or at the Pliocene–Pleistocene boundary.