The Privacy Commissioner wrote to Prime Minister Jacinda Ardern, pleading with the Government not to give Police a "blank cheque for information sharing".
In a letter, released by the Police, Privacy Commissioner John Edwards expressed his "significant concerns" regarding parts of the national firearms registry legislation, before it went before Parliament.
Specifically, he was worried about how the bill would have made it easier for government departments to share people's information with each other.
"In essence," Edwards said in the letter, which was dated August 5, "Police is asking ministers for a blank cheque for information sharing."
His lobbying appears to have been successful.
Police Minister Stuart Nash said changes were made to the draft legislation before it was introduced to reflect the issues Edwards raised.
A gun register is part of the second tranche of gun law reforms currently being voted on in Parliament.
In response to the March 15 terror attack, where a gunman took the lives of 51 people, the Government proposed creating a national register of firearms.
Although Edwards said he was supportive of sensible gun reforms, he said the proposed information-sharing system was inadequately thought through.
He said that it would unnecessarily compromise the security of very sensitive personal information and have unintended consequences.
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"My office has relayed concerns about the effect this proposal may have due to the potential number of individuals with access to the register, the potential for data breaches and the safety concerns surrounding a large number of individuals knowing who owns a firearm and where they live," he said in the letter.
In a statement, Edwards said he was concerned about the bill providing a large number of unspecified agencies direct access to the firearms registry.
He was also worried about providing health practitioners with the ability to confirm whether their patients hold a firearms licence.
Although the Prime Minister did not directly respond to the letter, the matter was dealt with by the Police Minister, Edwards said.
A spokesperson for the Police said that as the letter related to a bill which was now before Parliament, it would not comment.
But Edwards said the letter led to a discussion with the Minister of Police and improvements to the bill which mitigated many of the Commission's concerns.
The final version of the bill before the House shows the number of government agencies that would have access to the firearms register has been narrowed down to just the Ministry of Foreign Affairs, Customs and the Department of Conservation.
A spokeswoman for Nash said the minister was pleased the Privacy Commissioner raised these issues at an early stage so that he was able to sit down with him to resolve it.
Edwards was pleased with the changes.
"It is to the credit of both the Minister of Police and the New Zealand Police that they took on board my strong feedback and worked with me to find a way forward to deliver sensible gun reform in a more privacy-protective manner."