The man went to collect an agent appointment form, used to allow a person to act on another’s behalf, but was given an already completed copy from another client.
The seven-page document includes the full name, date of birth and signature of the client, as well as the name, date of birth, address, phone number, email and signature of their representative.
The man, who did not want to be named, said the breach of privacy was "unacceptable".
He was given the copy last month when he went to pick up the form to organise his mother’s superannuation.
He did not realise the mistake until he arrived home.
"At first I thought it was an example. Someone’s daughter had completed it for their mother."
He returned to Work and Income after the holiday break to hand in the form, but was denied access as he did not have an appointment.
"I didn't want an appointment. I just wanted to bring to their attention to a data breach."
He tried to explain what happened to the guard at the door and went to hand them the documents.
"The staff member raised their hands, stepped back and stated that they wouldn't be accepting anything from me if I didn't have an appointment.
"They just didn’t want to hear it."
He still has the forms and is unsure what to do with them.
Ministry of Social Development acting regional director Simon Rakiraki said an error was "very rare".
"We take our responsibility to protect the privacy of clients very seriously.
"We will be taking steps to minimise any risk of this happening."
The ministry was trying to contact the man to retrieve the document and "to thank them for doing the right thing by attempting to return it".
It also wanted to help him progress his own application.
"To protect the privacy and security of clients, our guards are not able to accept documents or paperwork under any circumstances."
The ministry would also be contacting any affected clients to apologise.
