An event-based record of 18O in precipitation at Margate in Tasmania, Australia, was analysed using 3d Lagrangian trajectories and composites of ERA40 850hPa geopotential height for the years 1994–2002. Trajectory analysis found that moisture entrainment occurs during the 48 hour period prior to arrival for all precipitating air masses at Margate. The rate of entrainment was greatest for events of high rainfall/high depletion, contributing up to 19% of vapour in the air mass in summer and up to 47% in winter. The majority of air masses were found to be advected from the Southern Ocean, however, for high rainfall/high depletion events during summer 30% of air masses were found to approach from the Tasman Sea (east of 155E longitude). Mean isotope ratios were less depleted by 1.4‰ for these events indicating mixing with less depleted air. High rainfall/high depletion events were also associated with negative geopotential anomalies east of Tasmania.