A Christchurch man says the Inland Revenue Department (IRD) mistakenly sent him a document with so many private details about a person that he could easily impersonate them.

The one-page document was buried amongst 200 pages that IRD sent to his address this month.

It included the first initial of the person's first name, their surname, IRD number, Work and Income client number and bank number.

It also included details of child support payments, the name of someone regularly putting money into their account, and the dates the transactions were made.

"The thing I find the most dangerous about it is I could ring Work and Income and say I was them and get it so I was receiving their Work and Income payments and child support payments, because all that info is right in front of me."

The man who received the document does not want to be named as he is the subject of an IRD audit.

Because of the nature of the document, which Fairfax has seen, it is likely that it was intended for someone who is also the subject of an audit.

He plans to file a complaint with the Privacy Commission about the breach, and says he would be horrified if his personal details ended up in the wrong hands.

"If it got into the wrong person's hands then you never know what's going to happen.

"I'm concerned about who might have what."

An IRD spokesman said the organisation could not comment on individual cases, citing confidentiality.

When the man initially complained to IRD about the privacy breach he says he was told that if he complained it would create more paperwork for the organisation.

The matter was then referred to Kiwibank, which wrote to him telling him IRD had accepted full responsibility for the mistake.

A letter addressed from an IRD manager says the organisation apologised for the error and an internal inquiry had been launched to ensure it didn't happen again.

It had also alerted the Privacy Commission.