We investigate sub-mesoscale circulations in an oligotrophic western boundary current, the East Australian Current, during a sustained offshore plankton bloom observed during an oceanographic field campaign. Using a series of high resolution hydrographic sections taken a few days apart, supplemented with in-situ samples of nutrients, satellite and long-term mooring measurements, we describe a dynamic situation by which the East Australian Current's core migrates zonally, moving approximately 100km over the course of 10 days. This current-core migration occurred concurrently with a sustained offshore plankton bloom. Upwelling motions on the inshore flank of the boundary current core occur concurrently with increased nutrient and plankton concentrations in the near surface. Calculations based on quasi-geostrophic theory suggest that these circulations arise from the interaction of a dipole-pair of mesoscale eddies with the East Australian Current, though strain induced frontal processes. Finally, using satellite data, we investigate the generality of these results. Assessing all offshore phytoplankton blooms identified by an automated method, we find very similar mesoscale patterns to those observed during the field campaign. We conclude that the conditions observed during the field campaign may have broad relevance to the physical and biological dynamics of the region.