A man was caught allegedly bashing a duck with a large piece of wood in the Halswell Domain on Thursday.
Police attended the incident on William Brittan Ave at 3.29pm, after it was reported by a witness that a man was harming a duck.
Constable Harriet Carey said they immediately stopped the man but the duck was seriously injured, so she removed her stab-resistant vest and boots and swam to the duck's aid.
"Its neck was bent at a funny angle and it was struggling to stay afloat. It was being supported by two other ducks that were using their beaks to stop it from drowning - I have never seen anything like it," Carey said.
She said she used bread grabbed from a nearby house to try and coax the duck over to the edge of the pond, but because of the trauma the duck had received it didn't move.