Personal details of firearms owners, including their names and addresses, were among documents allegedly stolen from the disused Auckland City police station, the Herald can reveal.
The documents could serve as a shopping list for criminals seeking to steal lawfully obtained guns.
A major criminal investigation continues to find out who may have come into contact with the private firearms documents.
Affected firearms owners are now being contacted by police and told their details were on the pilfered papers.
There has been no word on exactly how many firearms owners' details are caught up in the breach.
Officers became aware of the break-in while executing a search warrant in Mt Albert in May where they found police property, including documents and expired pepper spray.
Auckland City Police district commander Superintendent Karyn Malthus said four people have been charged to date with burglary and offences related to receiving stolen property.
"Among the stolen property recovered were a number of documents which included firearms licensing information, containing people's personal details," she said.
"Our investigation is continuing to work to understand the full breadth of documentation that was stolen.
"Police are now contacting people whose documents have been obtained as a result of the burglary to advise them of the situation.
"There is nothing to indicate any involvement of organised criminal groups in this burglary at this stage."
As part of the process following the major breach, police say they are working with the Office of the Privacy Commissioner.
Malthus said there was no evidence at this stage indicating the involvement of organised criminal groups in the alleged burglary.
"The ongoing criminal investigation is continuing to establish who may have come into contact with the stolen property.
"Police would like to reassure the public that this matter is being taken extremely seriously and the protection of all peoples information of the utmost importance which is why there is a significant ongoing investigation underway."
The Herald asked police whether firearms owners should consider taking additional steps to secure their weapons in view of the breach.
Malthus said: "In general, we always advise firearms license holders to remain vigilant around the security measures they have in place for their firearms."
Firearms owners were asked to contact police if they had any concerns.
The old Vincent St station was formerly the headquarters of Auckland City police before it was ditched for a new premises at College Hill several years ago.
Acting Privacy Commissioner Liz MacPherson said in a statement police would need to conduct an investigation to ascertain the size of the privacy breach.
"Any organisation holding personal information must protect the privacy and mana of the people who entrusted it to them."
MacPherson said while the Privacy Commissioner had not yet received complaints arising from the breach, anyone with concerns about their privacy could contact her office for advice and support.
The breach comes amid an ongoing gang war in Auckland where more than 20 houses have been shot up, some with high-powered rifles.
In this year's budget, police were allocated more than $200 million for the establishment of a new firearms unit.
Police are in the process of establishing a firearms registry, one of the recommendations of a review following the Christchurch mosque terror shootings.