The head of a review into New Zealand's intelligence agencies says much of their work doesn't need to be secret.

Former Deputy Prime Minister Sir Michael Cullen told TVNZ's Q+A that many of the documents and briefings he has seen could be made public, even if essential security information was redacted.

"And, indeed, I think the public would get a better idea of the need for the agencies if some of those documents were made public. I think, ironically, the agencies are their own worst enemy by being so secretive about almost everything that they do."

Sir Michael and lawyer Dame Patsy Reddy are heading a review of New Zealand's intelligence agencies.

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The review will examine the legislative framework governing the Security Intelligence Service (SIS) and the Government Communications and Security Bureau (GCSB), and whether current oversight provisions are sufficient.

Sir Michael told Q+A that one of the key issues to be determined was the definition of a "private communication", as he said the existing definition within the GCSB law had come under question.

"It is actually an extraordinarily difficult thing to define, both to ensure there's maximum protection for private communication and yet to enable some accessing of some aspects of that, should that be appropriate in the security context."

When asked whether he considered his own emails to be private, he said he wasn't sure.

"It depends how many people I've sent it to and what the context of that may be. But, for example, if I post on Facebook, I've got no expectation that that is private.

"So how do we cast the law in a way which copes with this changing kaleidoscope of privacy, which is indeed what we have in the present age?"

Sir Michael said New Zealand had to to share intelligence with other nations, as currently occurs through the Five Eyes arrangement, if it expected to receive reciprocal information about New Zealand's security.

He said the review would report back on the issue, "but inevitably, if we want information from offshore which is important to New Zealand security, which we can't collect all ourselves - it's completely beyond our capacity as a country - then we must expect there's some degree of reciprocity in that case".

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The review of the intelligence agencies is to be completed by February.