SPCA Otago executive officer Sophie McSkimming said the black and tan male puppy was found in the river by a fisherman yesterday.
"It's an absolutely horrific thing to happen ... We think it's been thrown off the bridge ... I can't even find the words to explain how upset I feel,'' she said.
"This poor little guy must've suffered.''
She hoped the distinctive lead which was used to tie the puppy to the rock might provide some clues about the identity of the person responsible.
"Someone must recognise this lead. It's quite distinctive and someone's got to know who had a puppy like this and no longer has it now.''
The matter was being investigated by SPCA inspectors and, depending on the outcome of the inquiry, the person or people responsible could face the most serious charges for breaking the law, Ms McSkimming said.
The puppy was believed to be purebred and a veterinarian who inspected the body thought it had been in the water for about 48 hours.
There were always other alternatives to killing a puppy, she said.
"The SPCA would've taken it ... There's no need to do something like this.''
"We try hard to educate people and teach them that things like this don't need to happen, that there's other options, but then someone goes and throws a puppy off a bridge to drown it ... It's hard to describe how upsetting that is. ''
• Anyone with information about the incident can contact the SPCA confidentially on 0800 682-467.
