The Accident Compensation Corporation says improvements to its information-handling processes will take time and it can not rule out the possibility of further privacy breaches.
This comes after it mistakenly sent a Waikato man details of another's criminal history - the latest in an embarrassing series of privacy gaffes by the corporation and other government departments.
ACC said it "regretfully acknowledges" the breach occurred, involving individuals of the same surname.
"The breach related to one page of information found in 3000 pages of requested material," it said.
The corporation has commissioned a three-year work programme to implement all the recommendations identified in the Independent Review of ACC Privacy and Security of Information report, released in August.
"While good progress is being made in mitigating risks, some errors may continue."
Bruce Liddington, of Te Awamutu, had requested ACC send through his personal files and details of a previous claim for his own records. The 39-year-old suffers from an old back injury having been in a motorcycle accident on a farm in 1989. He had another accident this year when he fell on his back and required surgery.
Mr Liddington was given 24 folders, containing details about his claims and injuries, and an extra piece of paper about another man with the same last name.
The document, seen by the Herald, includes the man's full name and date of birth, as well as four dates showing when and for how long he was sentenced to prison over a 10-year period. It also includes another sentence of home detention and remand dates.
Mr Liddington contacted ACC and was told someone would collect it.
"For weeks they told me they couldn't send me my files because they could get lost and therefore end up in the wrong hands - and then they go and send me the criminal history of some random bloke I don't know.
"I told them 'Don't you think this is a breach of privacy?' and they didn't give me an answer."
Privacy breaches this year
* ACC sends private information of 6500 claimants to former National Party insider Bronwyn Pullar.
*Ministry of Social Development kiosks at Winz centres are shut after sensitive information is accessed by a blogger.
*Winz staff member writes private information of clients on a scrap of paper and mistakenly passes it to a member of the public.
*Auckland DHB mistakenly sends Herald journalist private medical files.
*ACC sends Bruce Liddington details of another man's criminal history.