Determining the origin of individuals caught during a control/eradication program enables conservation managers to assess the reinvasion rates of their target species, and evaluate the level of success of their control methods. We examine how genetic techniques can focus management by distinguishing between hypotheses of ‘reinvasion’ and ‘survivor’, and defining kin groups for invasive stoats (Mustela erminea) on Secretary Island, New Zealand. 205 stoats caught on the island were genotyped at 16 microsatellite loci, along with 40 stoats from the opposing mainland coast, and the age and sex was determined for each individual. Using these data we compare and combine a variety of genetic techniques including genetic clustering, population assignment, and kinship-based techniques to assess the origin of each stoat. The population history and individual movement could be described in fine detail, with results indicating that both in-situ survival and breeding, and reinvasion are occurring. Immigration to the island was found to be generally low, apart from in one year where around 8 stoats emigrated from the mainland. This increased immigration was probably linked to a stoat population spike on the mainland in that year, caused by a masting event of southern beech forest (Nothofagus sp.), and the subsequent rodent irruption. Our study provides an example of some of the ways genetic analyses can feed directly into informing management practices for invasive species.