A man who cooked his pet dog in a backyard barbecue cannot be prosecuted because it was killed humanely, says the SPCA.

South Auckland man Paea Taufa was found roasting his pitbull terrier-cross in an umu pit at his home in Mangere, the Sunday News reported.

The dog had been skinned and gutted and was partially charred when SPCA inspectors arrived.

Mr Taufa said he had been surprised when inspectors arrived because dog was a delicacy in Tonga .

"I didn't know I couldn't cook the dog. In Tonga, any time there I cook the dog and it is okay. Dog is good food."

He had decided to cook the dog because it was too skinny and had become unmanageable. He rendered the dog unconscious with a blow to the head before slitting its throat, which is regarded as humane.

Under the Animal Welfare Act it is legal to kill a dog in New Zealand if the animal is slaughtered swiftly and painlessly.

However, SPCA Auckland chief executive Garth Halliday said Mr Taufa's actions were unacceptable.

"Although we appreciate the difference of cultures that exist in a place like New Zealand, the SPCA finds this sort of treatment of any animal to be totally unacceptable," he said.

"Even though the law says you can humanely kill an animal, you should not be treating any animal like this."

SPCA inspectors said there was evidence that killing and eating dogs was becoming more common in New Zealand.

Mr Taufa said he would not cook another dog.