Toxic algae has been detected in the Rai River in Marlborough and the Wakapuaka River in Nelson. Photo: Supplied
Toxic algae is making an appearance in rivers across the top of the South Island, with people told to avoid waterways where it is present.
The algae, benthic cyanobacteria, has been detected in the Rai River in Marlborough and the Wakapuaka River in Nelson.
It forms velvety, thick brown or black mats on rocks in the river bed, often along the water's edge and can produce toxins that are harmful to people and animals.
In Nelson, public health medicine specialist Dr Rachel Eyre said a health warning had been issued for the Wakapuaka River at Paremata Flats on Māori Pā Road after more than 50 percent cover of the potentially toxic algae was detected.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips."
People should avoid the area and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
Nelson City Council was monitoring the sites and the public will be advised of any changes in water quality.
Marlborough District Council environmental scientist Steffi Henkel said the council can't survey all of the region's extensive stream and river network for toxic algae - so there will not be warning signs in all affected areas.
Instead, people were being told to look out for the algae in rivers or streams and choose a different site if it was present.
In previous summers, the algae has been present in parts of the Wairau and Omaka rivers, which was likely to happen again this summer.
Anyone who suspects they have come into contact with toxic algae should visit their doctor and pets that show signs of illness should be taken to a vet immediately.