He never made it to his second stop as the poison set in.
"Not long after that they were walking towards the river and paralysis in his back legs caused him to collapse and it all happened really quickly. He was an otherwise healthy dog," Mrs Reynolds said.
Billy was rushed to VetEnt Havelock North and tended by veterinarian Heather Boaler.
"Some 10 minutes after being in this pond, and by the time I saw him, he was weak in his hind quarters and had vomited and subsequently went on to develop diarrhoea," she said. "His condition deteriorated overnight and due to his condition I recommended to his owner that the dog was euthanised for his welfare."
Mrs Boaler said she had done a lot of research since Thursday to better understand symptoms never seen in Hawke's Bay before.
"I haven't been presented with a dog with these symptoms in the area before and I just wanted to find out everything I could about the condition in this country," she said.
"In our opinion, and the symptoms and onset and apparent relationship to something that happened on the walk, do seem to fit the many aspects of the previously described cases of algae poisoning.
" There are so many different species of cyano bacteria and they all produce different toxins and different combinations of toxins and that's why it's very presumptive."
The regional council was notified of Billy's death and signs had since been erected warning users of potential dangers.
Resource Management group manager Iain Maxwell said the council were investigating the death, increasing testing regimes and warning the public to be wary around waterways.
"Staff have surveyed the Tukituki River in the area and confirmed there is low to no risk of cyano bacteria in the Tukituki River," Mr Maxwell said.
"Staff are now investigating the presence of toxins related to algae in the small stock watering pond implicated in the dog's death."
He said samples from the pond were being analysed to determine whether toxins related to algae were in the pond.
"Cyano bacteria is a naturally occurring algae species 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 this algae.
"HBRC has increased the frequency of monitoring in all rivers around Hawke's Bay in response to this incident."
He said owners should not let their dogs eat anything around waterways.
Mrs Reynolds also distributed pamphlets to neighbours and the clients of a nearby kennel.
"It's our first dog and the kids [aged 13, 15 and 17] are really upset and obviously it is a huge shock and I just want to make sure and hoping that no one else gets into the same position," she said.