A "completely defenceless" baby little blue penguin has died after being savaged by a dog.

The little blue penguin/kororā was taken to the New Zealand Bird Rescue Charitable Trust in Auckland's Green Bay with a bite under its flipper and a damaged right eye.

"What surprised us was the knowledge that the dog involved was not very large, judging by the bite size, and it was not the bite itself that killed," the trust said in a Facebook post.

"Within a week of being with us, both of the penguins' eyes were compromised and it had lost sight."

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The little bird was examined and it was determined it would have to be euthanised.

​The trust said the loss of sight had been caused by the penguin being shaken by the dog during the attack.

NZ BIRD RESCUE CHARITABLE TRUST The little blue penguin had a bite under its flipper - but that wasn't what killed it.

"It's been a very tough season of handling lots of baby and juvenile birds – this little fellow was nearly through his post breeding moult when attacked and they are completely defenceless while going through this process," the trust said.

"I guess the moral of the story is that even little dogs can cause damage to our precious taonga species.

"Please keep your dogs on lead even on the beach, especially during the summer months when breeding and moulting occurs."

The Department of Conservation said dogs should be kept away from dunes, vegetation, rocks or caves where penguins may shelter or burrow.

Little penguins are protected under the Wildlife Act, and owners of dogs who hunt or kill absolutely protected wildlife could face prosecution.

The charge carries a maximum penalty of two years imprisonment and/or a $100,000 fine.