Hot, dry conditions over the last few weeks mean that river conditions are ideal for the growth of algae including cyanobacteria. The brown or black toxic algae is visible as mats on the river beds.
These can wash off and build up on the river's edge. If ingested the algae can kill livestock and dogs and contact can cause vomiting, diarrhoea and skin irritations in humans.
Some dogs have a tendency to eat the algae so owners are warned to keep their dogs on a lead when near rivers.
Regional council senior environmental scientist Summer Greenfield said river conditions could change rapidly and as only certain points were monitored it is possible there are algae blooms happening at other spots.