Laboratory results have been unable to confirm whether a dog which died after a walk along the Tukituki River bank was affected by toxic algae poisoning.
But symptoms suggest it was the likely cause, a Hawke's Bay Regional Council (HBRC) spokesperson said.
Billy, owned by the Reynolds family in Havelock North, became ill after swimming in a pond next to the river last week.
He was euthanised the next day, as the effects of his illness became debilitating.
The dog's symptoms and history of contact with water suggested that cyanobacteria poisoning was likely, a HBRC spokesperson said.
However, an initial survey of the Tukituki River in the vicinity found low to no risk of cyanobacteria and test results from the pond were negative for algae toxins.
HBRC resource management group manager Iain Maxwell said while the results were negative in this case, algal mats found recently in other parts of the river provided a timely reminder for dog owners to be vigilant with their pets and not allow them to eat unknown material around the river to avoid poisoning.
He said the growth of potentially toxic blue-green algal mats is common in rivers during periods of lower flow and warm temperatures.
"Prolonged warm, dry weather combined with low river flows may mean that algal mats become exposed or easily accessible to dogs, stock and recreational users," Mr Maxwell said.
Hawke's Bay District Health Board medical officer of health Nicholas Jones said any contact with the toxic form of the mat could cause vomiting, diarrhoea, numbness or tingling around the mouth, and skin rashes in humans.
Dogs were particularly susceptible to the toxins which may cause paralysis, he said.
"Rivers users should avoid any contact with algal mats, avoid swimming in water where the mats are present, and keep dogs from scavenging around the river," Dr Jones said.
Cyanobacteria mats are identifiable by a strong musty odour and can vary in colour from brown/black when in the water to a pale brown/whitish colour when dry, he said.
Dr Jones said if anyone displays signs of illness after being in a river bed, they should seek medical advice from a doctor immediately.
Similarly, any dogs or other animals that become sick after being in a river should get prompt veterinary attention.
Cyanobacteria is a naturally occurring algae found in water bodies throughout the country.
As part of HBRC's routine water quality monitoring, staff run cross sections to check for the presence of the algae.