Hawke's Bay residents are being warned about potentially toxic algae surfacing along parts of the Tukituki River.
The algae is also appearing in the Ashley River near Rangiora in Canterbury.
Hawke's Bay Regional Council group manager Iain Maxwell says prolonged warm, dry weather combined with low river flows exposes the algal mats.
Mr Maxwell says dogs are particularly susceptible to the toxin, and owners should be extremely vigilant as any contact with the mat can cause paralysis.
Canterbury Medical Officer of Health Alistair Humphrey says exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
Dr Humphrey says those experiencing any symptoms should see a doctor immediately.