Since March 2025, South Australia has experienced an unprecedented, coast-wide harmful algal bloom (HAB) dominated by multiple Karenia species, including the brevetoxin-producing Karenia cristata. The bloom has affected >30,000 km² (>40% of the coastline) and caused extensive marine wildlife mortality. Analysis of multi-decadal environmental datasets indicates highly anomalous preconditioning in the bloom origin region, including several years of elevated chlorophyll-a and organic enrichment. These conditions were associated with record-strength coastal upwelling in 2024 and substantial Murray River flood discharge in 2023. Bloom initiation and intensification coincided with an unprecedented, prolonged marine heatwave, characterised by elevated sea surface temperatures, reduced wind forcing and enhanced water column stratification—conditions consistent with major Karenia events globally. Intermittent low-level Karenia detections over the past decade suggest a persistent, cosmopolitan species pool primed to respond when conditions aligned. Our synthesis indicates the 2025 HAB emerged from the convergence of multiple interacting drivers rather than a single dominant cause, and highlights key priorities for improved observing systems, quantitative attribution and predictive modelling to support future risk management.