Beginning in March 2025, South Australia experienced an unprecedented, coast-wide harmful algal bloom (HAB) dominated by multiple Karenia species, including the brevetoxin-producing Karenia cristata. Affecting more than 20,000 km² and approximately 30% of the state’s coastline, the event caused extensive mortality of fish, sharks, rays, invertebrates and other marine fauna, and placed significant pressure on fisheries, aquaculture, public health systems and coastal communities. The response required rapid mobilisation of science, monitoring, policy and industry partnerships under conditions of substantial uncertainty. This presentation describes how early community reports evolved into a coordinated, evidence-based response that integrated species identification, molecular diagnostics, oceanography, remote sensing, seafood safety testing, ecosystem impact assessment and recovery planning. Key advances included the expansion of statewide surveillance from fewer than 10 initial sites to more than 300 monitoring stations, deployment of an array of near-real-time oceanographic moorings, development of rapid qPCR assays and DNA metabarcoding approaches, establishment of laboratory cultures of bloom-forming species, and the progression of local accredited brevetoxin testing capability. Integrated modelling, satellite products and field surveys provided situational awareness and supported decision-making for public communications, fisheries management, aquaculture activities and ecological recovery. The response also highlighted the importance of citizen science and industry partnerships, alongside national and international collaboration. While attribution remains complex, current evidence indicates that the harmful algal bloom arose from the convergence of multiple interacting physical, chemical and biological drivers, rather than a single cause. The South Australian HAB response provides a case study in building adaptive capability during a prolonged marine environmental emergency and demonstrates the need for sustained observing systems, rapid diagnostics, cross-sector governance and long-term ecological recovery planning to improve preparedness for future HAB events.