When new individuals from a pest species are detected after an eradication programme, it is important to determine if these individuals are survivors from the eradication attempt or reinvaders from another population, as this enables managers to adjust and improve the methodologies for future eradications and biosecurity. Rangitoto/Motutapu Islands in the Hauraki Gulf (New Zealand) had a multispecies mammalian pest eradication conducted in 2009. A year after this eradication a single stoat was trapped on the island. Using genetic population assignment we conclude that this individual was a reinvader, which probably swam a minimum distance of 3 km from the adjacent mainland. This swimming distance is greater than any previously known stoat incursions. Our results suggest that the original population on these islands was from natural dispersal rather than anthropogenic introduction and that it had some limited ongoing mixing with the mainland population. These findings highlight the invasion/reinvasion potential of stoats across large stretches of water, and will necessitate ongoing biosecurity indefinitely for these islands. The study also highlights the utility of genetic assignment techniques for assessing reinvasion, and emphasizes the need for pre-eradication genetic sampling of all pest species to enable such analyses to be carried out.