Treasury Secretary John Whitehead says the department has to raise its game after a second blunder over lost documents this month.
In the latest incident, thousands of personal and company tax details on a computer disk, were lost.
Two weeks ago, an official's notebook with information about a possible merger of intelligence services was found on the street.
Mr Whitehead told Morning Report he was not happy about the breach and said there will be a review.
He said the person responsible did not follow the correct prodecure, but their actions were out of character.
Mr Whitehead said there were no personal details on the disk.
