Prospective pet-owners are warned to look out for "puppy scams" this Christmas, with a growing trend of people selling dogs that do not exist.

Wellington police issued the warning after noticing an increase on fake trading websites.

Investigation Support Unit's Constable Kylee Coubrough said the fake websites had been set up to sell puppies sight-unseen.

"At Christmas time, we sometimes think we can get good deals through online traders and it's not always the case," she said.

"Police have noticed an increase of people selling puppies which don't exist."

"They can be sought-after and popular breeds such as English bulldogs, huskies, Staffordshire bull terriers to Maltese and even toy poodles."

People should avoid transferring any money unless they have seen the dog, Police say. Photo / 123rf

She said sellers often claim puppies are potty trained and full vaccinated and often claim to have their own delivery service, so buyers cannot view the dog before purchase.

"These sellers are also making 'change of ownership' documents for each purchased puppy, which is not a thing in New Zealand," she said.

"Neither is a breeder registering the puppy before you get it ... all these things are red flags."

She said the public needed to be aware of this trip and to do extensive research before paying anyone for a puppy.

People should always buy from a reputable breeder, and ask to see the puppy before exchanging any money, including deposits.

"Trust your instincts, if it sounds like a scam, and it's too good to be true it probably is."

To report information on illegal activity, people can call the Police non-emergency number 105, or dial 111 in an emergency.