The threat imposed by invasive species and difficulties associated with control and management places more impetus on trying to prevent their introduction. The identification of introduction pathways is a vital component towards this goal. In this study, we use a genetic marker-based approach to retrospectively investigate the pathway of origin of the invasive Argentine ant (Linepithema humile) into New Zealand. We intensively sample the mitochondrial gene cytochrome b, from the entire known range of Argentine ants in New Zealand. No genetic variation was found in New Zealand. In order to identify likely introduction pathways, we use two alternative genetic analyses and suggest that a tcs approach that collapses identical haplotypes and calculates the probability of parsimony is superior to standard phylogenetic tree-building algorithms. A minimum spanning network allowed relationships to be examined among sequences collated from previous international studies. The cytochrome b sequence, when compared to a global database, matched that from an Australian population. That Australia is the potential source of Argentine ants is in agreement with the New Zealand interception record, as goods from Australia have the highest number of interception records of Argentine ants. Our approach can easily be duplicated for other organisms and the methodology can be more widely applied to help aid further efforts to identify the routes of transmission for other invasive species and allow us to efficiently direct our biosecurity monitoring effort.