Dog owners in Manawatu are being warned about the presence of potentially toxic algae in the region's rivers and streams.
The Manawatu-Wanganui Regional Council says says cyanobacteria, or blue-green alga, is becoming a problem as river and stream water levels fall.
It says the alga, which is typically found on stony beds or beaches, can cause skin irritations in humans and can kill dogs.
Environmental scientist Kate McArthur says significant amounts are in the upper Manawatu River.
Ms McArthur says dogs are at high risk from the algae, and should be kept well away from any areas with obvious growths.
"I didn't expect the growths to be anywhere near as extensive as they were.
"Everywhere upstream of the Manawatu Gorge, every shallow point in the river was pretty much covered in black phormidium growth."
Ms McArthur says dogs are at high risk, and should be kept away from any areas with obvious growths.