Cyanobacteria, or toxic algae, are naturally present in all New Zealand waterways.

During hot, dry weather, toxic algae blooms can form in rivers, making swimming sites dangerous for people and dogs.

These blooms create black, green or brown slime on rocks, or brown or black ‘mats’ at the river’s edge that have a velvety texture and musty smell that can attract dogs to lick or eat them.

Toxic algae is harmful to humans and dogs when ingested; a piece the size of a 50c coin is enough to kill a dog.

Council monitoring has found moderate levels of cyanobacteria throughout the Maitai River, including popular swimming and dog walking areas at Avon Terrace, Dennes Hole and the Maitai Campground.

Nelson Marlborough Health and Council recommend that people and pets stay out of those areas of the river to avoid the potential contact with cyanobacteria mats while warnings are in place.

Warning signs are up at affected areas. If you see toxic algae mats, avoid contact and keep your dogs and children out of the water.

What to do:

  • Keep your dog on a lead and out of the water around affected sites.
  • If you think your dog has eaten toxic algae, contact your vet as soon as possible as the toxins can take effect within minutes.
  • If a person experiences a reaction to toxic algae, seek medical help immediately.

For more information go to the Nelson City Council website.