A toxic algal mat on a rock from a river. Photo: Supplied / Otago Regional Council
Dangerous levels of toxic algae have been found in the Hutt River, north of Wellington.
The regional council said the public should not swim between Māoribank Corner and Taitā Rock, where large numbers of detached algal mats have been spotted.
Dogs should be kept on leashes and away from the river's edge.
The Greater Wellington Regional Council advised that anyone who came into contact with toxic algae and was feeling unwell should contact their doctor immediately.
Anyone who believed their dog may have swallowed toxic algae should take it to their nearest vet immediately, the council said.
The Greater Wellington Regional Council made a video outlining what toxic algae looks like and discussing its dangers back in 2017.
The council said during summer low rainfall and higher water temperatures created an environment which allowed toxic algae to thrive.
Toxic algae ranged from blackish brown to dark green in colour and looked different to normal algae which was bright green and often formed along streams, the council said.
Toxic algae mats could come loose and wash up on the river's edge or form floating blocks in shallow areas, the council said.
As the mats dried out they turned light brown or white and produced a strong musty smell which was when there was the greatest risk to dogs, the council said.
"They love the smell and many dogs will try to eat it if they get the chance," the council said.
That could be extremely dangerous as a piece the size of a 50 cent coin was enough to kill a dog, the council said.
People should go to the Lawa website to know if it is safe to swim, and seek medical attention for you or your pets if any algae is swallowed.