Until the early 1980s, marine HABs were considered to be a problem mainly of the Northern Hemisphere but thought to be virtually non-existent in Australia. We now know of species that: (1) generate essentially harmless water discolorations, affecting tourism (Noctiluca, Trichodesmium), but occasionally can lead to indiscriminate kills of fish and marine invertebrates from oxygen depletion (Sydney Harbour 1890); (2) kill fish, but have no human health significance (Chattonella, Heterosigma, Prymnesium, Karenia mikimotoi); (3) produce potent neurotoxins that can cause gastrointestinal and neurological illnesses in humans (Alexandrium; Gambierdiscus; Gymnodinium catenatum, Pseudo-nitzschia cuspidata); and newly emerging: (4) species that cause catastrophic ecosystem disruptive blooms (Karenia cristata). Recurrent questions with every HAB event are: is this a natural phenomenon, or have anthropogenic influences, such as eutrophication, climate change, aquaculture activities, or human mediated translocation of alien species contributed to its development? Effectively managing of HABs requires conclusive species taxonomy; toxicology and ecophysiology from cultured strains (mixotrophy); rapid detection tools (e.g. qPCR, ELISA); long-term field data collection (e.g. IMOS) for modelling; and ultimately assessment of HAB impacts against risks from mitigation strategies (e.g. clays, bubble curtains). The recent South Australian HAB has highlighted critical deficiencies in preparedness frameworks at state and national levels.