Police have revealed the details of around 400 gun owners - including names and possibly the types of firearms they had - were stolen from the old Auckland police station.
Sensitive documents were being stored inside the dilapidated and disused former Auckland City police station where staff no longer work.
"We probably have a few hundred firearms licence holders' details in the documents that we have recovered," said Superintendent Karyn Malthus, Auckland District Commander.
She said the details of around 400 people have "potentially" been taken.
The documents had been stored at the old station since last year in a bin destined for the shredder.
The documents were supposed to be destroyed, but never were. Police only realised they were stolen after finding them during an unrelated search of property in the Auckland suburb of Mt Albert.
It has upset gun owners.
"It's most disappointing that the police are almost casual in the way they leave material like that around," said Sporting Shooters Association of NZ president Chaz Forsyth.
Spokesperson for the Council of Licensed Firearms Owners Hugh Devereux-Mack is also upset.
"This makes firearms owners feel incredibly unsafe because the police have let us down in a way that puts a target on our backs for organised crime."
Police are reviewing their systems and said staff are gutted it occurred.
"And we will work through with them to understand what occurred that enabled this but I can assure you that they're mortified that documents have been stolen," Supt Malthus said.
The stolen documents relate to firearms owners or those getting renewals between 2015 and 2017 in central Auckland.
Expired pepper spray was also taken, along with police uniforms and copper piping in what police are calling an "opportunistic" break-in.
Four people have been charged.
"Look I think this is a significant learning moment for our people," Supt Malthus added.
Supt Malthus wasn't certain if every one of the documents had been recovered, saying investigations were ongoing.
But Forsyth worries such a breach could occur with the new gun register.
"This is why we believe it's a waste of public money which could be better spent elsewhere in preventing crime."
But police reject this and said the chance of paperwork being stolen will be greatly reduced as the new gun register will be a digital system.