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.
Constable Carey swam across to get the duck.
"Turns out all those years I did lifeguarding finally came in handy," Constable Carey said.
Police then took the duck, now referred to as Daisy, to the Bird Hospital and are waiting to hear what the outcome will be for her.
Police said the man is known to them and has been taken into custody.
Inquiries are continuing.