A memo advising HealthCare New Zealand staff to be careful with client files was among more than 50 patient health records and documents found on a Christchurch street.
The documents were found on the street in Merivale on Saturday afternoon after they were taken from a HealthCare NZ employee's private car which had been broken into about three weeks earlier.
The person who found the documents handed them onto regional television channel CTV and Mainland Press.
The records mainly belong to elderly patients, and include information such as medical dosages and prescriptions, but also notes from staff on how to access some clients' houses and the hours they are normally home.
HealthCare NZ is a community-based health care provider which offer support for elderly and disabled people, and is contracted by the Canterbury District Health Board.
CTV said it contacted HealthCare NZ and the CDHB earlier this week, who were unaware of the breach. HealthCare NZ and police are now investigating.
HealthCare NZ community services manager Scott Arrol told CTV the breach of client confidentiality is a "major concern".
"We're really sorry about the event. It's not the sort of thing that happens to us," Mr Arrol said.
"These sorts of things can happen to any sort of organisation, the process we have in place now is about identifying what's gone on, what we can learn from it and how we can improve."
HealthCare NZ is looking into why it took so long for the documents to turn up after the car was broken into, but it does not believe the information was used by whoever broken into the male nurse's car.
A wallet, GPS equipment and a stereo were also stolen from the car.
"We believe that as a result of breaking into the car and going for the belongings they've just come across this information and dumped it," Mr Arrol said.
Also among the items stolen were medical manuals, text books and internal memos, including one advising nurses to be careful with client files, suggesting they be locked away in the car's boot when being taken out for assessments.
HealthCare NZ is this week contacting the clients concerned.
Privacy law specialist Jonathan Forsey told Radio New Zealand the loss of the records was a gross breach of privacy.
He said standard codes of practice for storing and transferring documents were set out in the health information privacy code, and those protocols could have been breached.
"So I don't think that the significance of this can be underestimated - it's not just pieces of paper with words written on it. It's actually vitally important information and in some instances it has commercial value.''
Mr Forsey said it was unusual for medical information to be transferred in a private staff car.
The Canterbury DHB's general manager of planning and funding Caroline Gallery said the only protocol that was breached was a delay in reporting the documents were missing.
She said it was normal for staff members to take information with them when they visited clients.
Earlier this month, HealthCare NZ was selected as one of six providers for ACC's home support services.
ACC has itself been responsible for a large breach of client details.
Last year the details of 6748 clients - including nearly 250 claimants who were the victims of sexual or violent assault - were emailed to another ACC client, Bronwyn Pullar, who exposed the breach.