The willow sawfly, Nematus oligospilus Förster (Hymenoptera: Tenthredinidae), has been introduced inadvertently across temperate regions in the Southern Hemisphere, including New Zealand and Australia, where it has dispersed extremely rapidly. A host specialist herbivore, at high population densities it can defoliate and damage introduced willows (Salix spp.), many species of which are invasive weeds in Australasia. In this study, we show that in just 10 years, the distribution of N. oligospilus has expanded across most of south-eastern Australia and south-western Western Australia and, in less than 15 years across both the North and South Islands of New Zealand. The relative population density of N. oligospilus varied widely within both geographic regions. The distribution of N. oligospilus was still expanding in Australia where high densities resulted in widespread willow defoliation. In contrast, the distribution of the willow sawfly had reached fully across both the North and South Islands in New Zealand, but its density had declined from the early stage of invasion. All willow taxa surveyed were utilized by the willow sawfly, but host preferences were apparent, especially for the S. fragilis/S. x rubens hybrid complex. Many factors, including natural enemies, willow range expansion and environmental conditions are likely to affect the long-term population dynamics and spread of N. oligospilus in Australia.