Up to a third of the pets Tauranga vets see are obese, they say, leading to shorter life expectancy, lower quality of life and diseases like arthritis and diabetes.
Tauranga's Holistic Vets owner Dr Liza Schneider said the carbohydrates found in commercial pet foods were largely to blame for the country's overweight cats and dogs.
New Zealand pets were being fed a lot of the wrong types of food, setting them up for diseases like obesity, diabetes and skin conditions, she said.
"There's definitely been an increase (in obese animals).
"The modern pet society is overfed and undernourished."
Cats and dogs were carnivores, designed to survive on meat and protein.
Dr Schneider suggested feeding dogs a balanced diet of raw pre-frozen meat and raw bones.
High-fat treats were also a major contributor to obesity with a 25g piece of cheese equating to the human calorific equivalent of one-and-a-half hamburgers to a dog and two-and-a-half hamburgers to a cat.
Pets were just as happy to eat a smaller treat that took them the same time to devour as a larger piece, she said.
Dr Schneider also recommended pets be given a good quality Omega 3 fatty acid supplement, which helped the body to burn excess fat.
Co-owner of Bayfair Vets Steve Alderson said about 30 per cent of his cat and dog patients were obese, mostly as a result of being given treats and table scraps.
"Where most people go wrong is feeding tidbits and treats," he said.
Giving your pet some attention - a pat or a run in the park - were healthier alternatives for rewarding pets, he said.
Another option was to save some of their daily food to feed to them as a treat between meals.
Pets carrying extra weight led to shorter life expectancy, increased arthritis and diabetes. "They don't have the same quality of life. They just don't have so much fun," Dr Alderson said.
Heart and respiratory complications could also result from pets being overweight, he said.
At Tauranga Vets Dr Holly Rabone said owners often overlooked the fact their pet may be entering senior years and needed a lower fat diet.
"You wouldn't feed your grandmother the same as your growing teen," she said.
Dr Rabone said there were some "grossly overweight pets", and quite a few moderately overweight at the clinic. Vets could put pets on special diets that were lower in calories and higher in fibre, filling them up and stopping them pining for food. Increased exercise was also important as well as taking the pet for weekly or fortnightly weigh-ins at the vet's.
Meanwhile, local butchers are struggling to meet customers demand for healthier pet food. At Bostock Butchery in Te Puna, owner Garth Bostock is selling hundreds of gluten-free dog rolls a week and 40kg each of gluten-free offal and rice, and chicken and rice dog stew.
"My dog sits at the freezer as soon as he hears my car coming home," he said.
At Cherrywood Butchery and Wholesale in Otumoetai, manager Shane Snowdon said the butchery's gluten-free pet mince was a big seller, as was the butchery's possum meat dog roll.
A lot of owners didn't like their pets eating foods packed with additives, he said.