Police say there has been a potential privacy breach involving the online platform for the firearm buyback programme.
The web page allows people to notify police they have a prohibited firearm or related item.
"Immediately upon being made aware of the issue the platform was closed down and we are investigating the matter further," a police spokesperson says.
"We have advised the office of the Privacy Commissioner of the potential issue."
The Council of Licenced Firearms Owners told Newshub that information on 70,000 firearm hand-in notifications, the firearms and owner bank account numbers, was accessible to web page users.
"They were able to screenshot and download information. This means that gang members or other criminal elements could have accessed this information before our supporters found the breach," a spokesperson says.
"Keep an eye on your bank account - possibly notify your bank to stop unusual transactions. You may consider whether you should take extra personal and home security precautions.
"This is exactly what we feared of an incompetent agency in charge of an online register."
The Government banned many military-style weapons in the wake of the March terror attack in Christchurch, which cost 51 people their lives. Owners have been able to hand them over in exchange for cash.
But the buyback has been plagued by controversy and protests by gun owners. At a protest last week, organiser Stewart Hydes said gun owners are being made out to be criminals and being punished for something they did not do.
"The police are to blame for failing to properly vet the Christchurch gunman," he told Newshub.
"Policy advisors and legislators are to blame for failing to make sure the firearms he used were not as heavily restricted as other military-style semi-automatic firearms were."
The Mongrel Mob has also hit out at the Government's changes, saying they were "racist" because police were getting access to more weapons and they - a predominantly Māori gang - weren't.