Health warnings have been issued near several popular South Island camping spots after a toxic algal bloom was discovered in nearby rivers.
The bloom has the potential to harm people and animals that come into contact with it.
Annette Scott has been holidaying on the banks of the Hurunui River for 18 years and on a hot Canterbury day, she wasn't letting a toxic algae warning put her off taking a dip in the water.
"We look for ourselves and we can see that it's lovely and clear and it's flowing - no, we're not worried at all," she says.
But authorities say a danger does lurk below the surface, with tests revealing high levels of potentially toxic cyanobacteria phormidium.
"It can give you feelings of nausea, it can give you strange tingling sensations around your lips, and peculiar sensations that can lead to you needing to go to a doctor," says Canterbury Medical Officer of Health Dr Alistair Humphrey.
It can also have fatal consequences for dogs that eat it, which is why further north near Nelson, Helena George was taking no chances, keeping her dog on a leash and out of the Maitai river where the toxic algae has also been discovered.
"It's a real shame when you think about New Zealand 20 or 30 years ago and you could swim in the rivers," she says.
"They were such a nice community thing to have, so it's such a pity."
Environment Canterbury (ECAN) scientists have been testing the region's rivers every week and say the "mat-like" algae now covers 20 percent of the Hurunui river bed.
"We see it at this time of year because it tends to grow in warmer water temperatures and when there's long stable periods of flow," says ECAN water quality scientist Dr Adrian Meredith.
So for now the warnings will stay in place, until the affected rivers are given the all clear.