A former top spook has revealed a secret of the craft - join the Koru Club.

Sir Bruce Ferguson, former boss of the secretive Government Communications Security Bureau, said he was told years ago the best way to find out what was going on in Wellington was to join the Koru Club, then spend a couple of hours there on a Thursday or Friday night.

"You have a whole raft of politicians and businessmen coming through, most of whom are so important they must talk to either the person next door or very loudly on the phone."

In a surprisingly candid speech to the internet forum NetHui in Wellington yesterday, Sir Bruce also lifted the lid on how the GCSB believed it could get away with spying on New Zealanders for so many years, despite legislation specifically banning it from doing so.

He confirmed that about 50 of the 88 cases identified in an inquiry earlier this year as potentially breaking the law against spying on New Zealanders had happened under his watch.

"I received a warrant signed and duly checked by the inspector of warrants and the head of either the police or Security Intelligence Service and the boss [the prime minister]. It comes to me and it asks specifically for help from the GCSB to spy on a specific target . . . they have to convince me in that warrant the reason why they're doing that and that means they have to show they have reason to believe that person is acting against the security of the state.

"They then have to ask by name for the people in GCSB who may be able to help them - the specific specialist by name. I then sign a warrant or agreement in that warrant to second those individuals. I second them to the asking authority, be it the SIS or the police. So I've now seconded them, as far as I was aware, to that organisation.

"They go across there, they do what is required by the SIS or the police and they finally finish the task and come back. At no stage . . . was I ever aware or made aware of the outcome. That wasn't my business."

Sir Bruce admitted he had reservations initially.

"I must admit early on I thought is this right or not? But I was assured it was. That is why this particular Act has to be clarified. Not one of those 88 [people] was actually outside the law, assuming the secondments were correct. But OK, there is so much grey area there that law must be followed up."

But Sir Bruce said while he agreed the law needed to be fixed, he opposed the speed with which the Government was rushing the change through.

"We must get the entire bill correct, get the oversight correct, get the right people . . . anything that goes through fast you've got to be a little worried for."