A 5-year-old girl helped save her baby sister from a frenzied pitbull attack in Whangarei - with the help of her injured pet dog.

The Saturday attack happened as John Williams, his wife Stacey Waipouri, her 5-year-old daughter Bella-Rose Waipouri-Globbits, and the couple's 8-month-old daughter Alaska-Dawn Williams-Waipouri were having lunch.

The pitbull slipped under their fence and attacked the family's 9-year-old huntaway bitch Kim in the porch outside the open door to the kitchen.

The huntaway retreated into the kitchen, but once she realised the baby was on the floor she returned to the doorway and the pitbull's jaws.

Kim gave loud, agonised howls as she was bitten and Mr Williams grabbed "the biggest knife you've ever seen" to stab the pitbull eight times.

"Blood was going everywhere, but it never yelped and it never flinched. It just kept going and going for my dog," he said.

Young Bella-Rose plucked Alaska-Dawn from the floor and carried her to safety in a bedroom, shutting the door.

"I was frightened, but when I saw Alaska near the blood, that's when I knew I had to take her away," Bella-Rose said.

The pitbull dragged Kim from the kitchen to the front of the house.

Ms Waipouri was bitten when she tried to help Mr Williams separate the animals.

A man visiting his wife's parents nearby came to their aid and gave the pitbull two cracks on the head with a heavy piece of timber. He kept a foot on the dog's head until police and an SPCA officer arrived.

Mr Williams said he had never seen an animal so determined to kill.

SPCA duty officer Sunel Ferreira took both dogs away for examination by a veterinarian.

The pitbull was put down because of its wounds and the huntaway returned home after treatment for bruising and bites.

Ms Ferreira said the pitbull had no collar or other means of identification.