Cookie the dog looked more like the Cookie Monster when his owners first laid eyes on him.

Weighing in at an impressive 63kg, the huge, fluffy Labrador had started to become a spectacle at the SPCA when Auckland couple Arthur and Jenny Chin arrived to adopt a new family pet.

"He looked more like a polar bear," Mr Chin said. "His ears were stretched and looked small compared to his gigantic frame.

"The first time he sat in his new kennel, we thought it would collapse under his weight."

Mr Chin said although they were shocked when they saw the oversized Labrador, something "clicked" and the couple knew that he was the one for them.

A strict diet of biscuits and fruits was immediately on the cards for Cookie - who was first called Norm, but was renamed Cookie by the Chins for being a softie.

"We enrolled him in a [diet] programme that year and it was something that motivated him and us too," Mr Chin said.

"We got him walking and swimming - he loves to swim - at least five times a week.

"At first he'd start panting after walking only 15 metres. He wanted to play, but there was only so much we could do."

Cookie was among dozens of overweight dogs put under the SPCA Hill's Slimmer of the Year campaign.

After more than 18 months of being on a special diet - which included biscuits, apples and blueberries - and going for his regular swims, what once looked like the Cookie Monster had trimmed down to a healthy dog weighing around 40kg.

Cookie was announced the winner of the competition and was given a year's supply of dog food.

However, his owners - wanting to give something back to Cookie's former home, the SPCA, and also not wanting to tempt their pet too much - decided to donate the food back to the animal welfare group.

Mr Chin said the new and lighter Cookie, who regularly chases birds around their property now, is not only a much healthier dog but also a more happier one.

But his appetite is still as big as ever.

"He loves fruits - especially blueberries," Mr Chin said. "But he also loves the odd cheeseburger ... but only for a treat."