Prime Minister John Key says it was a Treasury official who dropped a notebook, containing secret information, that was later found by a journalist.

A review of the way the Security Intelligence Service, the External Assessments Bureau and the Government Communications Security Bureau work was revealed in a notebook found by a Radio New Zealand (RNZ) reporter.

Mr Key confirmed the review was being carried out by former foreign affairs secretary Simon Murdoch.

"It's very high level. He will report back to me, it's too early to tell whether there will be any changes," Mr Key said.

The notes set out options for the way the security services operated and one was a merger, RNZ reported.

"We are looking at the structure of the three intelligence agencies, whether there's greater synergies and whether they can work together more.

"But I haven't even had any advice, in fact the journalist that has the notebook has more information on the outcome of those meetings than I have."

The review was part of the Government "running the ruler" across a number of areas, Mr Key said.

He would not be drawn on what other reviews may be underway out of the public eye.

The notebook was found in a Wellington street by reporter Julian Robins.

Mr Key said the situation was "less than ideal".

"I think it's regrettable but people do make mistakes.

"It was a simple error, the person left it behind or lost it somewhere."

Mr Key said he would not want to see the owner lose his or her job but any consequences were an operational matter.

The loss was reported to Treasury.

Treasury secretary John Whitehead said he had apologised to Mr Key about the incident and it was being followed up internally.

He confirmed the notebook belonged to a Treasury staff member and arrangements were being made for its return.

Robins said the notebook contained information across a range of issues, not just the review of the security intelligence services.

He said he would give it back to its owner.

Labour leader Phil Goff said the information was not a national security issue.

"So it's not at the top end of seriousness. It is careless, it will be embarrassing to the person who lost it," he told reporters.

"The information could have been of a far more sensitive national nature."

He expected the person would be reprimanded but said it appeared to be a case of human error.

Mr Goff said the idea of a merger had not been looked at by the Labour government.

He is on Parliament's top secret Intelligence and Security Committee, and while the proposal was news to Mr Goff he expected the committee would have a chance to consider any recommendations.