The tropical moist margin occasionally extends into the extratropics, where it can lead to heavy precipitation in otherwise dry regions. Here, we examine the physical processes involved in these events and their interaction with the extratropical flow. Employing tools such as budget and trajectory analysis, we analyse a case study of an extratropical interaction with the moist margin in January 2018. This event produces a subtropical cyclone and results in damaging winds and precipitation over New Zealand. The event is then simulated with the ACCESS-rAM3 model, highlighting the critical role of latent heating in the development of the cyclone. Overall, the results suggest that interactions between extratropical PV and the moist margin occur in both directions. Deep cyclonic PV anomalies of upstream extratropical origin can result in poleward moisture advection leading to poleward displacement of the moist margin, while convection inside a perturbed moist margin can lead to upper-level ridge building through poleward advection of anticyclonic PV by the divergent wind. These results have implications for the diagnosis and prediction of severe weather, especially for extreme precipitation in the subtropics.