The dog, thought to be no older than 2, was taken to a vet and his skull X-rayed
The worst was confirmed.
It appeared the bullet had shattered inside the dog's skull which was causing extensive neurological problems.
Another piece of a bullet was found in the skin of the dog's jaw and the vet estimated the shooting had taken place a few weeks earlier.
It was decided the best option was to euthanise the dog as the damage was extensive.
Ms Vezey said she took the fragment to a firearms expert who said the bullet had come from a gun used for possum or rabbit shooting but was not powerful enough to kill a dog.
"It seems like he was deliberately executed.
"But they didn't make sure he was dead when they walked away. He had to suffer," Ms Vezey said.
"If you are going to kill an animal make sure they are out of their misery and you use the right firearm."
She urged owners of unwanted animals who did not have the means of rehoming or disposing of them humanely to contact either the SPCA, a local vet or council, to see what options were available.
"It's better than an animal suffering or being prosecuted," she said.
Ms Vezey had been back to the area where the dog was found and interviewed locals and asked for information about the dog.
Anyone with information about the shooting should contact the Bay of Islands SPCA office on 09 407 7515.