Police and the SPCA are investigating the shooting of 33 puppies and dogs on a rural property in Wellsford, north of Auckland.
Senior Constable Barry Rose says officers arrived at Rowan Hargreaves' property on Tuesday to find a pile of dead puppies, a mesh cage full of dead dogs, some in a van shot through the head and other dogs down the driveway.
Mr Rose says the dogs were not killed humanely. Police have seized a shotgun and are investigating whether firearms charges should be laid.
The SPCA says it hopes to lay charges of wilful cruelty against two men believed to be responsible for the shootings. The charge carries a maximum jail term of three years.
A man who witnessed the shootings and tried to stop them described the scene as "a bloodbath".
Richard Hawkings, a friend of Mr Hargreaves, says the incident began when one or more dogs belonging to the owner were believed responsible for killing a neighbour's fox terrier.
Mr Hawkings says Mr Hargreaves shot two of his own dogs out of respect for his neighbour's loss and offered to shoot several more because he felt bad.
He says two of the neighbours came over with a form requesting permission for them to shoot the animals and Mr Hargreaves allowed for some of them to be killed.
However, Mr Hawkings says the men then opened fire on the dogs, killing every one in sight.
"It became a bloodbath. The guys ... started firing shots left, right and centre at these dogs and in any direction. They didn't care whether they hit, or whether they were gonna hit," Mr Hawkings says.
"It went from a culling to a slaughter to a massacre. I was crying - I threw a bottle to try and snap them out of it. They were fixed in their mind to kill every dog right down to a six inch puppy."
Dogs endured pain and suffering - SPCA
The SPCA says two inspectors visited the property on Wednesday and an initial investigation indicates the dogs endured pain and suffering throughout the ordeal.
SPCA staff broke down in tears on Thursday as they recounted details of what is being described as 20 minutes of frenzied shooting.
Inspector Sacha Keltie, the first to arrive at the property, says it was "a haunting scene of death and destruction".
Auckland SPCA executive director Bob Kerridge says in his many years for the organisation, he has never seen an atrocity against animals on such a scale as this.
Rodney District Council spokesperson David Anderson says its officers were called out to a complaint about the number of dogs on the property and whether they were registered, but by the time they arrived, the dogs had been killed.