Australia’s Integrated Marine Observing System (IMOS) operates one of the world’s most comprehensive long-term plankton observing programs, combining high-frequency net and bottle sampling at National Reference Stations with extensive oceanic coverage from Continuous Plankton Recorder surveys. These observations have delivered nearly two decades of consistent, open-access data, transforming our understanding of phytoplankton and zooplankton biodiversity, biogeography, and ecosystem responses to climate in Australian waters. This talk synthesises these scientific advances and their management applications. We show how plankton data have revealed large-scale shifts in plankton communities linked to climate change, documented ecosystem responses to heatwaves, and enabled discovery of fundamental macroecological relationships. Plankton observations have supported national ecosystem assessments by providing Essential Ocean Variables that quantify plankton biomass and diversity, and by delivering key ecological indicators of climate change, ocean acidification and ecosystem health. Plankton data have been used to validate biogeochemical and ecosystem models, in Australia and globally. The physical sample archive has enabled novel discoveries using both traditional taxonomy and molecular tools. Finally, we share lessons from sustaining a long-term plankton observing system, offering insights for emerging programs. As the time series grows, the IMOS legacy will continue to increase in value for marine science, management and policy.