A dispute between two neighbours has led to the slaughter of 33 dogs.
Some were maimed first in the "bloody, rifle-killing frenzy" before being "finished off", the police say.
Others were shot inside their cage or hit by ricocheting bullets. Still more were shot at point-blank range.
The youngest were 3 weeks old.
The SPCA is investigating whether the animals were killed humanely, or if charges should be laid.
The dispute, on a rural road in Wellsford in Rodney District, unfolded on Monday when a fox terrier belonging to Russell Mendoza was discovered dead after apparently being mauled by another animal.
Police say he blamed one or more of the 39 dogs owned by his neighbour, Rowan Hargreaves.
Mr Hargreaves, who kept the animals on his 5ha block in an old quarry, did not believe they were to blame.
However, a day earlier he had shot one after discovering it had attacked a sheep.
He said he had never before had any trouble with his dogs getting out.
Mr Hargreaves, a mechanic who lives alone in a broken-down truck surrounded by car wrecks, said that after Mr Mendoza buried the fox terrier, he and another man arrived, armed with a .22 rifle and a 12-gauge shotgun.
They handed Mr Hargreaves a note for him to sign saying he agreed to their shooting the rest of his dogs.
He said he signed the letter because he felt "under enormous pressure".
He then stood behind a shack to shield himself from the "bloodbath".
Yesterday, holding back tears, he described the sounds of his dogs being shot - sounds that echoed off the quarry walls for 20 minutes.
"They 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."
In all, 23 pups and young dogs, which slept in Mr Hargreaves' truck, were shot, as were a male and female dog living in a van wreck and eight adult dogs housed in a kennel. They were shot through the grating.
Four pups hiding under their mother in the van survived, as did two other dogs the shooters didn't see.
These six were taken to Mr Hargreaves' workshop in Wellsford, but one later died. None of the dogs had been registered.
Mr Hargreaves' business partner, Richard Hawkings, watched the killings but was powerless to help.
The SPCA took away some of the slain animals, and yesterday Mr Hargreaves was digging a grave in the quarry to bury the rest.
Mr Mendoza would not speak to the Herald, on legal advice.
Senior Constable Barry Rose said he found 66 rifle cartridges and seven shotgun shells on the property. He could see dozens more among the dead dogs, which were "piled on top of one another" in the corner, where they had cowered to avoid the gunfire.
"I think [the gunmen] went absolutely ballistic and let everything go until the last one stopped squealing.
"I would liken it to a gung-ho on a bloody, rifle-killing frenzy.
"I've been in this job for over 35 years and this is the first time I've ever come across something like this."
Mr Rose said one of the shooters did not have a firearms licence. A decision had yet to be made on whether he would be prosecuted.
Police had received no complaints about Mr Hargreaves' dogs, he said.
Neighbours the Herald spoke to had not heard of any dispute between Mr Mendoza and Mr Hargreaves, and none had had any trouble with the dogs. The animals, all mongrels, were the offspring of a single breeding pair.
Mr Hargreaves had recently arranged to drop several off to the SPCA to be housed elsewhere.
Asked why he had had so many dogs, Mr Hargreaves said they were "my family".
SPCA executive director Bob Kerridge said two investigators had visited the property and would determine whether the dogs suffered before they died. A decision would then be made on whether to charge the gunmen. Wilful ill-treatment carries a penalty of up to three years' jail.