Harmful algal blooms (HABs) are major threats increasing in frequency and intensity worldwide, and are responsible for the mass die-off of many marine species due to oxygen deprivation, toxins, and changes to the water permeability. South Australia experienced an unprecedented HAB in 2025, affecting a broad range of elasmobranch (sharks and rays), with washups of southern fiddler ray (Trygonorrhina dumerilii) and Port Jackson shark (Heterodontus portusjacksoni) being among the top 20 species reported on iNaturalist’s SA Marine Mortality Events 2025 – 2026 page (2,333 and 817 observations, respectively), and with mobile threatened and protected species also being impacted, i.e., nine white shark (Carcharodon carcharias) mortalities were recorded during peak HAB. We use a multi-disciplinary approach combining toxin analyses, fisheries-independent surveys (baited camera, diver survey), and acoustic tracking to identify the physiological mechanisms through which HAB event impact elasmobranchs, compare the impact of the HAB across species, quantify declines in local populations, and monitor recovery. Deployments of 480 baited cameras at 20 sites and tracking from 51 acoustic receivers along Adelaide beaches showed a complete disappearance of benthic elasmobranchs during the peak HAB events, with a near nil recovery 10 months post-peak. Diver surveys found no new egg cases at known egg-laying sites suggesting that the HAB event led to some species, e.g. Port Jackson sharks, either laying eggs at a different, potentially less favourable site or skipping a breeding season. Our study informs governments of the impacts of algal blooms on elasmobranchs, likelihood of local depletion, and will support future active recovery, e.g., rewilding, if necessary.