New Zealand's spy agencies are to come under greater government oversight, but information about their operations and co-operation with foreign agencies will remain closely guarded secrets.

Prime Minister John Key yesterday said the Cabinet had decided to improve "the effectiveness and governance of the New Zealand intelligence system".

The changes followed a review of the agencies by former head of Foreign Affairs and Trade Simon Murdoch and were "designed to ensure the agencies keep pace with changes in the security environment, so we can better meet the security challenges we face".

The review found the NZ Security Intelligence Service (SIS) and the Government Communications Security Bureau (GCSB) were too fragmented and did not operate together as efficiently as they could.

However, Mr Key yesterday said the review "found no performance failure at agency or system level".

The announced changes would give the Department of the Prime Minister and Cabinet, the Treasury and the State Services Commission greater oversight of the intelligence agencies' performance, priorities and resources.

However, those central government departments would not have any role in overseeing the actual operations of the intelligence agencies.

While the agencies' operations are under the oversight of the Inspector-General of Intelligence and Security and the Commissioner of Security Warrants, Green MP Keith Locke - and other critics - says their activities are largely determined by overseas agencies.

New Zealand MPs were "refused basic information such as how much Waihopai costs each year, let alone what it's doing", he says.

"Even the Prime Minister's office hasn't been able to control very much what they do."

Meanwhile, Mr Key also said the director of the National Assessments Bureau would now be responsible for a national assessments programme that would include domestic and external intelligence sources.

Mr Key told TVNZ News last night the increased focus on domestic threats "doesn't mean that we mightn't from time to time undertake activities which are in the interests of other countries.

"You've got to have that balance right between your international and your domestic operations."

TVNZ's copy of the Murdoch review had material detailing the extent of co-operation with overseas agencies and the capability of New Zealand's agencies removed.