"I had to decide whether to chase him or double-check exactly what I'd seen him put in the inlet," father-of-two Mr Beaumont said.
"He saw me taking photos and turned his face away.
"I got out of van to confront him and he bolted across the park without saying anything.
"Later on I was 95 per cent certain what I had seen was a puppy, but I began wondering what if it had been a baby?"
Now, Christchurch police have posted the photos on their Facebook page in a bid to identify the man.
The page has sparked a vigorous discussion about what it's alleged the man has done.
Many have expressed anger at the purported crime, while others urge caution that the man could have legitimate answers for what reportedly happened.
"Here's hoping none of you ever come up before the courts and have a pre trial via Facebook to judge you guilty before the police get to talk to you," one poster said.
Others wrote of their concern at what Mr Beaumont saw.
"People that harm small fluffy innocent animals are mental cases and should be locked up as if [sic] they hurt a human," one said.
Another added: "Even if you didn't kill [it] why the heck dump it in a damn river ... not hard to give it a kind send off scum bag!"
A police spokeswoman confirmed they were investigating.
Mr Beaumont has been left shaken by the event.
He described the man as being "very casual, nonchalant ... which is probably another alarming factor".
"I was pretty traumatised by it," the painter and decorator said.
"I went back to work but only lasted an hour before I went to the police station to make a statement. I was pretty cut up about it."
