Science has not yet identified when Phormidium turns toxic or why, so to be safe it should always be treated as harmful. Contact with Phormidium bacteria can make people sick, with flu-like symptoms.
Dogs are more vulnerable. They are particularly attracted to the musty odour of the mats and the toxins can be more dangerous. Dogs should be under control in river beds and not allowed to scavenge, and owners should take other water for dogs to drink.
Symptoms are vomiting, diarrhoea and lethargy, and heat stroke-like symptoms such as muscle tremors, breathing difficulties, paralysis and convulsions. Dogs should be taken immediately to the vet.
Algae and phormidium occur naturally, even in very clean fresh water. As regional council staff cannot check whole river systems each week, they appreciate information and photos from the public. Phone 0800 108 838 or email photos to info@hbrc.govt.nz (subject: Algae alert).
Check water quality and algae warnings on the national environmental monitoring site LAWA (Land Air Water Aotearoa) at lawa.org.nz, searching Can I Swim Here.