The national archive has been hit by an apparent security breach and has shut down the online search tool the public uses.
An investigation is under way into whether restricted records have been mistakenly made publicly discoverable.
The national archives is the government's record-keeping authority and has more than seven million records.
Archives NZ discovered last week that some restricted data was popping up in public view when it should not have been.
"On Friday, 11 November, Archives NZ discovered a potential security and privacy issue with its Collections records management system [Collections search] that required it to be taken down from public view," chief archivist Anahera Morehu said.
"Archives NZ discovered that a set of records, with brief record titles containing private information, was publicly visible, despite the records themselves being restricted and not visible."
However, Friday's occurrence was not the first problem with the new Collections search system.
"In recent months, we have observed that some records or information associated with records, are appearing in unexpected places," Morehu said.
"This could, potentially, have security and privacy issues which is why we have needed to make Collections unavailable to the public for periods of time."
One user told RNZ the new search tool was "dreadful" and slow.
Archives NZ drew RNZ's attention to Morehu's statement only after RNZ lodged an OIA request on Tuesday about the search tool.
The agency got the system from vendor Axiell, and together they were now working to make sure this did not happen again.
Until it knew for sure, the Collections system would remain down, Morehu said.
Friday's problem - with records that are not available digitally and have never been accessible to the public - was "a continuation of a series of issues".
"The Department of Internal Affairs is aware there have been performance issues with Archives NZ's Collections Search tool since it went live in February 2022," Morehu said.
"We remain committed to improving the Collections search."
The contract with Axiell covered maintenance and improvements.
"We expect our provider to work with us to ensure we have an archival information management system that makes it easier for our staff and users to access collections.
"This, clearly, is not currently the case."
Archives had held workshops to hear back about users' experiences, and was providing updates.
It looked at backtracking to use its old system, but could not.
"Unfortunately, Archway had reached the end of its life, with the technology no longer maintained, supported or able to be improved.
"This was a major risk to the security of our data and the prompt for us to move to a new system."
Archway had not been updated since the new search tool went live so the data it holds was now months out of date.