South Australia's 2025 mass fish mortality event was driven by a widespread Karenia Harmful algal Bloom (HAB), reported as the largest and longest-lasting HAB event recorded in Australian waters. The bloom affected more than 20,000 km² of coastal waters and was associated with catastrophic marine life losses, raising immediate concerns for fisheries, aquaculture, and marine ecosystem health. In response, the South Australian Research & Development Institute (SARDI) rapidly established a broad scale, multi-agency, phytoplankton sampling regime to track bloom extent and intensity while supporting risk assessments under evolving conditions. This was achieved through coordination across fisheries teams and partnerships with commercial charter vessels, enabling broad spatial coverage of offshore waters. Concurrently, targeted training enabled in-house capability to consistently identify and quantify Karenia, ensuring timely and reliable data generation. The monitoring program provided critical information to track bloom dynamics across South Australian waters and support fisheries risk assessment, aquaculture management decisions, and operational responses. In addition to quantifying bloom extent and intensity, the program provided field data to ground-truth satellite-derived bloom products, collected samples from impacted areas for culture establishment, and generated material for molecular analyses to support species identification and future HAB research capability. We demonstrate how cross-sector partnerships, flexible sampling platforms, and rapid capacity building can underpin effective monitoring responses to HAB events associated with large-scale marine mortality, providing a practical framework for fisheries and aquaculture preparedness during future "summer of extremes.