Dog owners are being warned to keep their pets on leads after toxic algae was found in parts of the Hutt River near Silverstream.
Wellington Regional Council environmental science manager Graham Sevicke-Jones said it was potentially fatal, so dogs should be kept on leads and people also needed to take care.
Monitoring had shown the amount of brown or black toxic algal mats growing on the riverbed had reached moderate levels.
The algae could kill livestock and dogs, and cause vomiting, diarrhoea and skin irritations in people if ingested, Mr Sevicke-Jones said.
The recent spell of fine weather had caused the river to drop and, combined with warm temperatures, that meant ideal conditions for algal growth.