It kills dogs and is dangerous to humans, but 1 NEWS has found swimmers aren't being warned about a toxic algae deemed unsafe by authorities.
Parts of Canterbury's Selwyn River are in red alert mode due to dangerously high levels of the algae.
But most families, some with their pets, cooling off in the river at Glentunnel over the warm weekend were completely unaware of the risks.
The potentially toxic algal blooms, called cyanobacteria, can make people very sick.
And Environment Canterbury has detected it along sections of the Selwyn this summer at Glentunnel and Chamberlains Ford.
"If you ingest them, strange and unusual effects are produced. They might get tingly feelings in their fingers and their legs or they might get unusual rashes or cramping in their stomach, vomiting and diarrhoea," said Dr Alistair Humphrey, Canterbury Medical Officer of Health.
No one has died in this country from the anatoxin the bacteria produces, but many pets have.
"It does kill dogs. We've had at least 100 dogs killed by these algal blooms," Dr Humphrey said.
The only signage 1 NEWS could find at the river's entrance was one saying the algae may be present and to look out for it.
But the sign signalling the water as 'high risk' was 200 metres away, on the road.
"There is a second sign closer to the water edge, but clearly it's not being seen by people when they're in there. We're open to improve and put additional signage if that'll help get the message through," said Douglas Marshall if Selwyn District Council.
If you're unsure whether to dive in, the Land Air Water Aotearoa website lists up-to-date data on just how safe New Zealand's swimming spots are.