Our understanding of the physical factors driving fine-scale structuring of marine biodiversity remains incomplete. Recent studies have hypothesised that oceanography and coastal geometry interact to influence marine biogeographic structure on small spatial scales. The coastal waters of eastern Tasmania, located at the oceanographic interface between two major boundary current systems (the East Australia Current (EAC) and the Leeuwin Current (LC)) represent an informative system for assessing this hypothesis. Parallel biogeographic and oceanographic analyses, focusing on the relative abundance of two widespread, larval-dispersed Nerita gastropods, suggest that the relative influences of the EAC and LC at this interface are modulated by coastal topographical variation. Specifically, east-facing coastal sites are dominated by the EAC-derived N. melanotragus, whereas south-facing bays are dominated by LC-derived N. atramentosa. These combined oceanographic and biological data imply that coastal topography and hydrodynamics can combine to influence the local distributions and abundances of planktotrophic-developing taxa at coastal convergence zones.