The 33 dogs shot dead in what police have called a "bloody, rifle-killing frenzy" would have cowered in fear, says SPCA head, Bob Kerridge.
The dogs were shot following a row between neighbours on a rural property near Wellsford, North of Auckland.
Police and the SPCA were both investigating the shootings and Mr Kerridge said the dogs would have been terrified.
"It was a total massacre."
The dogs were shot after a fox terrier belonging to Russell Mendoza was killed and he blamed the death on or more of the 39 dogs kept on his neighbour Rowan Hargreaves' property.
The next day Mr Mendoza and another man arrived at his property with a .22 calibre rifle and shotgun and handed Mr Hargreaves a note saying he agreed to the shooting of his dogs, the New Zealand Herald reported.
Mr Hargreaves said he signed the note because he felt under "enormous pressure".
The two men then began firing at the dogs, he told the newspaper.
He said the dogs were screaming, "making sounds dogs just don't make. When one was gone, the others knew they'd be next, but they had nowhere to go".
Mr Kerridge said animals instinctively knew when they were in danger.
"They may not know the purpose of it and they may not know what was going to happen but they are certainly aware of danger and there is no question they would have reacted in that way.
"You have not only got the aspect of the firing, you have got animals dying around them and you have got the sound. From what we understand the sound was horrendous of these dogs crying in agony. That will communicate a message to all the other dogs, without a doubt."
Mr Kerridge, SPCA's executive director, said he had been promoting animal welfare for many years but when he heard of the slaughter he was "absolutely amazed. I have been in the game for a long time but we have never had anything like this."
Some of the dogs were shot in their cage or were hit by ricocheting bullets while others were shot at point-blank range. The youngest were three weeks old.
Mr Kerridge said the SPCA was still investigating whether anyone should be charged over the incident.
Police are also investigating the shooting, including the possibility one of the gunmen did not have a firearms licence.