By RNZ
The Manawatū-Whanganui regional council said its staff test waterways regularly.
It asked people to stay out of any rivers with black or dark-brown slimy mat-like growth on the stones, which could be musty smelling - and to keep pets clear too.
In lakes, the algae form bright green "blooms" which can form surface scum at the lake edge.
The cyanobacteria that causes toxic algae blooms can cause skin irritation, stomach cramps, vomiting and nausea in people who ingest it or come into contact with it, and can kill dogs.
