More than 75 firearms were seized during a recent police operation in Bay of Plenty and Gisborne, while 50 firearms purchased suspiciously in 2022 remain unaccounted for.
In a statement, police said three firearms licence holders and one revoked former licence holder have been charged with unlawfully possessing firearms, and a firearms dealer's licence has been suspended.
Two people have pleaded guilty to the charges.
"Our investigation identified the men, aged in their twenties and thirties, made suspicious purchases of a large number of firearms in the Western Bay of Plenty and Auckland over a four-month period in 2022," police said.
"It is particularly concerning that a firearms dealer is part of this investigation ... Firearms dealers are subject to higher thresholds of conduct to be considered a fit and proper person to hold a dealer's licence."
Court appearances will begin in early September, with a reappearance in Gisborne on September 8, two sentencings in Whakatane on October 9, and a hearing in Opotiki on October 30.
Police said they suspect the 50 firearms not found are now in the underground trade.
"The majority of firearms licence holders are good law-abiding people, unfortunately there are still a few who divert firearms to unlicensed offenders.
"We know from the recent review of the Firearms Registry that firearms diversion from licence holders to the black market remains a threat to public safety."
That review found that the registry, launched in June 2023, had only stopped one firearm from being sold to an unlicensed owner, and that there is still a need for further action from the top.
It nevertheless found that the registry was making a difference by easing resource usage and speeding up the investigative process.
"The registry is cost-effective and has wider financial benefits by reducing the amount of Police resource required to investigate firearms diversion."
Current regulations require that all firearms be registered with the Firearms Safety Authority by end of August 2028.
Police said that anyone with concerns about a potential firearms offence can phone police on 105, or 111 if life is in immediate danger.
