Contact with Phormidium bacteria can make people sick, with flu-like symptoms.
Dogs are more vulnerable - they are particularly attracted to the odour of the mats and being smaller, the toxins can be more dangerous.
Dogs should be under control in river beds and not allowed to scavenge.
Clinical signs are vomiting, diarrhoea and lethargy, and heat stroke- like symptoms such as muscle tremors, breathing difficulties, paralysis and convulsions.
Warning signs would be put up at affected areas by the Hawke's Bay District Health Board, but people were urged to avoid contact with the Cyanobacteria mats and to keep animals away.
In January a no-swim warning was put in place for parts of the Tukituki River when Cyanobacteria mats became exposed.
- Check water quality and algae warnings at lawa.org.nz, or for more information go to lawa.org.nz/explore-data/hawkes-bay-region/river-quality/tukituki-river/tukituki-river-at-black-bridge-(haumoana)/