The people of Australia are very familiar with toxic cyanobacterial blooms, as they have been a long-standing problem for agricultural and human drinking water supply, as well as for the recreational use of water. Livestock poisoning by cyanobacteria was first described in the last century near Adelaide, and the names of water-courses such as \'Poison Waterhole Creek\' across the country reflect the hazard from cyanobacteria. More recently 1,000km of the Darling River carried a massive bloom of PSP containing Anabaena which killed an estimated 10.000 livestock and required emergency water supplies for several towns.This year (and last) in the centre of the City of Adelaide the Torrens Lake (no longer used for water supply) had a heavy bloom of toxic Microcystis, with waterfowl deaths. While livestock poisoning is relatively common,cases of human and wildlife poisoning are however rare, more through avoiding drinking evil-smelling water than to an absence of toxicity in cyanobacterial blooms.Effective assessment of the risk to human health requires data which relate the dose of toxin to the clinical effects in a population. When ,in the past, an adverse health effect from a cyanobacterial bloom has been observed in the population, no measurements of toxin in the water supply have been made. Even in the recent case of deaths of 50 dialysis patients in Brazil ,the best that could be achieved was retrospective analysis of post-mortem samples for toxin. As a result animal toxicity data are used for risk assessment, incorporating safety factors, to derive Guideline Values for a safe water supply.WHO has just announced the first of these values for cyanobacterial toxins, for the toxin microcystin,of 1.0 mg/litre of water.The other major potential hazards in water supplies in Australia are cylindrospermopsin from the tropical Cylindrospermopsis, and PSPs from Anabaena. All three toxic cyanobacteria occur in water supply reservoirs as intermittent blooms, often controlled by the supply authority by copper sulphate application.This lyses the bloom liberating toxins free into the water.Children tend to be more vulnerable than adults to toxins in the drinking water, partly because they have less choice of what they drink.Blooms usually occur in summer,when water consumption is high and swimming is popular.These hazards are recognised,bloom warnings are given on the radio and by erection of signs by rivers and lakes.Monitoring for blooms in reservoirs and rivers is essential to reduce the risks to the population and is undertaked widely in Australia.