A prisoner took a muster sheet containing the details of 52 other inmates out of jail, and posted it on Facebook.
Milton prison in South Otago learned of the latest public sector privacy breach last weekend, after discovering the inmate had managed to sneak the sheet out when he was freed.
The Dominion Post can also reveal a second muster sheet from a different prison was then found in the cell of a Milton prisoner, raising concern about the security of the documents.
A muster sheet contains a mugshot, full name and the cell location of each prisoner in a particular area of the prison.
It is used by staff to track prisoners' locations - for example, marking when they are on particular duties such as kitchen work.
It is understood that, since details change regularly, new sheets are often printed and sometimes placed on top of the old sheets on a clipboard.
In an email to staff on Tuesday, Milton prison manager Jack Harrison said the prison was notified last weekend about the publication of the muster sheet, which appeared to be from late July or early August.
The privacy commissioner and the ombudsman were informed and all 52 prisoners, some of whom had been freed or transferred since the date of the muster sheet, would be informed in writing of the breach, he said.
"For now and ongoing, please please ensure you treat the information we have as part of our daily working routines as confidential and think about what it would look like on the front page of a newspaper or on a TV screen - my experience is that our muster sheets don't make for good TV."
In a separate email, meant for other prison managers throughout New Zealand but accidentally sent to all staff, Mr Harrison cautions his peers that the breach could happen anywhere, as a muster sheet from another prison had just been found in the cell of a prisoner who was transferred to Milton.
"We are in the spotlight for the wrong reasons and, while this would be bad at any time, recent privacy breaches across govt depts makes the timing as bad as it could get, I'm picking."
The breach is the latest in a string of bungles plaguing government departments' handling of people's information.
Earlier this year, a file containing thousands of ACC claimants' information was mistakenly sent to a client, while this week it was revealed papers with personal information were wrongly given out by Work and Income staff.
A Corrections spokesman said it was believed the Milton muster sheet was from a low-security area, but it was still unknown how it had come into the possession of the prisoner.
A complaint had been made regarding the theft with police, who were investigating, he said.
The department would also conduct its own inquiry.
It was a mystery how the second muster sheet, from a North Island prison, had managed to make it into a cell in Milton, he said.
Prisoners are searched on entering and leaving prison, and processes had been reviewed after the incidents.
It had not been decided whether the department would also write to the 52 prisoners, or apologise to them, as it was yet to be determined if the department was at fault, he said.
A spokesman for the privacy commissioner said no complaints had been received to date.
- The Dominion Post