The spokeswoman said if you, your family or pet comes in contact with the toxic algae medical assistance should be sought.
"The main risk comes from humans touching it, or dogs eating it. If your child becomes ill after going to the river it's best to take them to the doctor, and to take dogs to the vet," she said.
Dog deaths have occurred due to phormidium poisoning, according to HBRC – the Tukituki river is prone to this algae because of its stony environment.
The council has released a dog owner's guide to algae in rivers.
Phormidium occurs in shallow river and looks "like a blackish mat that forms on rocks", the spokeswoman said.
It has the potential to produce toxins and is a form of cyanobacteria – blue-green algae.
Under certain conditions, this algae has the potential to release toxins into the water at concentrations high enough to be a risk to dogs and humans.
The HBRC have told dog owners to look for the warning signs and to check social media for updates and news on the state of the region's rivers.
The spokeswoman also said to be vigilant about letting dogs or children scavenge at the water's edge if it is expected that phormidium might be present.
She said if the 'black mat algae' peels off the rocks and floats to the riverside it can pose a serious problem.
"If enough of these mats collect at side of rivers, there is a risk that dogs will eat these potentially toxic organisms," the spokeswoman said.
"Dogs are attracted by the musky smell given off as the mats decay."
