Thalassiosira blooms off Eden, New South Wales, are not unusual and have often clogged plankton nets. The Australian Continuous Plankton Recorder Survey has been operating for 15 years and shows that large blooms of the common diatom Thalassiosira, less than 20 µm in size, can be expected between July and August in this region. We encountered a bloom during a voyage on RV Investigator in July 2023, whilst sampling for zooplankton in cool, nutrient-rich, low oxygen and salinity water being injected onto the continental shelf. The bloom was identified as Thalassiosira partheneia and contributed greatly to the biomass sampled. T. partheneia, a small centric diatom of 8–13 µm diameter, has good food value for grazing zooplankton, although the irregular gelatinous masses it forms can be difficult for zooplankton to process. The threads produced by this species and its aggregation into gelatinous masses are thought to increase sinking rates leading to high carbon export, despite small cells classically thought to be entirely remineralised within the microbial loop. With the zooplankton composition being dominated by small, herbivorous copepods and microzooplankton, what can we learn about how this bloom species affects productivity?