The New Zealand screwshell Maoricolpus roseus was unintentionally introduced to south-eastern Tasmania in the 1920s. It has colonised more habitat than any other high-impact benthic marine pest in Australia and its wide temperature and depth tolerance makes further spread likely. We developed three sets of genetic probes, each targeting a unique region in the mitochondrial COI locus, for the rapid detection of this species in mixed plankton samples. In particular, we wanted to know whether this species has a planktonic life-history stage that could lead to its dispersal in ships’ ballast water. All probe sets were tested against as many closely related species as could be obtained and the reaction conditions were optimised for maximum sensitivity and specificity of M. roseus. Plankton samples collected in the Derwent Estuary between August 2003 and June 2004 were tested with the probes using a nested polymerase chain reaction. Maoricolpus roseus was detected in the plankton samples especially in the spring–summer period. The presence of M. roseus in the plankton and water available for ships’ ballast indicates that the risk of this species being spread by shipping needs to be managed.