A toxic algal mat on a rock from a river (file photo). In lakes, free-floating algal blooms tend to form, Health NZ said. Photo: Supplied / Otago Regional Council
People should avoid contact with the water at Ōkawa Bay in Lake Rotoiti, due to a potentially toxic algal bloom, Health New Zealand says.
The agency said people should not to swim in Ōkawa Bay or do any recreational activity that could result in significant contact with the water.
Algae blooms are when large amounts of algae multiply and grow, particularly in warm weather. Blooms of blue-green algae can release toxins into water.
National Public Health Service medical officer Dr Lynne Lane said those toxins could affect the nervous system, causing numbness, difficulty breathing and asthma attacks.
Contact with the water could also cause skin rashes, stomach upsets and visual problems.
"Algal mats and scum may build up along the edges of lakes or rivers, so it's especially important that parents ensure that children avoid contact with these as they may be toxic," says Dr Lane.
"You should seek medical advice from your GP if you become unwell after having contact with the recreational water."
Algae blooms can be harmful to dogs, as well as people.