A duck rescued by a police officer after a brutal bashing may have suffered serious brain damage.

Officers arrived at the pond at Halswell Domain in Christchurch on Thursday to find an elderly man bashing a duck with a large piece of wood.

The duck, known as Daisy, was rescued with bruising and abrasions to her dorsum and right wing. She was weak and wobbly on both legs and might have neurological damage, a police spokeswoman said.

CANTERBURY POLICE Constable Harriet Carey rescues Daisy duck from a savage beating at a pond in Halswell Domain in Christchurch.

A volunteer at the Christchurch Bird Hospital was caring for her and a vet would reassess her condition on Tuesday.

The offender was charged with cruel ill treatment of an animal. He was due to appear in court in Christchurch in a few weeks.

Steve Brown, who was at the lake as the leader of the Strikeforce Boxing Canterbury children's group, said he called police after an altercation with the man who was "killing ducks in the most inhumane, disgusting way".

JOHN KIRK-ANDERSON/STUFF Strikeforce Boxing Canterbury founder Steve Brown was at Halswell Domain lake with a group of children when he found a man killing ducks and called police.

"A couple of the little 7-year-olds said they heard a real commotion in the bush and thought it was ducks fighting, but I could tell by the noise someone was hurting them.

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"I went over and the man had a duck by the neck shaking it and it was just flapping around and screaming."

Brown said he man, who was dressed in blue overalls similar to those worn by caretakers at the park, eventually dropped the bird but showed "no remorse, no apology, no embarrassment."

"He just said, 'That was a tough one, it got away'."

Brown said it was common for people to usher ducks away from the pond as it was a popular area for miniature trains and boats, and he had once heard a man telling children to throw rocks at ducks to get them away from the area.

He called police, who arrived about 30 minutes later to find the man assaulting another bird.

"We immediately stopped him and the duck appeared to be seriously injured. Its neck was bent at a funny angle and it was struggling to stay afloat," Constable Harriet Carey said.

"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.

"My main concern at that point was the welfare of the duck."

Carey tried to lure the duck to the water's edge with scraps of bread, but it refused to budge so she put her years of experience as a lifeguard to use.

"Fair play – it probably didn't particularly like humans anymore. I had to remove my stab resistant vest and boots and swim across and get it. And yes, it was freezing."

SPCA Canterbury chief executive Barry Helem said reported cases of duck abuse were rare, but in the past four years there had been two reports of geese and swans in the Avon River area with arrows or bolts protruding from them.

There was also a case of two swans being shot in residential Christchurch. People needed to respect the ducks and "just leave them alone", Helem said.

"They will move on when they are ready. We do not advocate throwing anything at ducks as this will only stress them and risk causing injury. We also advise not to feed ducks or ducklings as this will only encourage them to stay."

Killing mothers was particularly concerning as orphaned ducklings were unlikely to survive without intervention as they were at high risk of being killed by predators or traffic, Helem said.