The spook who stumbled on the biggest spy scandal in New Zealand history, Kit Bennetts, is as convinced as ever that top civil servant Dr William Sutch was a KGB agent.

Mr Bennetts was a raw 23-year-old on routine surveillance one night when he followed a Mercedes from the Soviet Embassy in Wellington and witnessed a KGB officer meeting an unknown man, later identified as Dr Sutch. Other secret meetings followed before Dr Sutch was apprehended one rainy night.

Thirty years on, the former Security Intelligence Service officer has written a book about his role. Dr Sutch was acquitted in 1975 on a spying charge and died seven months later.

The release today of Spy marks the first time an SIS officer has gone public with details of the shadowy work of the security service. Until now, he was known only by his trial name, Mr S.

The SIS said it had "no comment" about Mr Bennetts or his book. A spokeswoman for Prime Minister Helen Clark, who is the Minister in Charge of the SIS, said yesterday: "The book does not have SIS approval, so it is not an authorised account."

Mr Bennetts, who left the SIS in the mid-1980s, worked for two overseas intelligence agencies and is now a police officer in Brisbane, said yesterday that Dr Sutch was a paradox.

"He was a loving husband. He was a great father. He was a great family man. His role in the social development of New Zealand was great. Many would say that would outweigh this silly little dabble with the Soviets, whereas I say he was involved in a full-on intelligence operation as an asset of the KGB. To me that outweighs the good he did.

"I honestly believe he never saw himself as a traitor. I don't think he would have done anything to consciously harm New Zealand. The fact that he did is probably a product of his arrogance ... and his belief that he perhaps knew better."

Mr Bennetts considered writing a book about the case in 1991 but backed off after running the idea past the SIS.

Even now, with the Cold War over and the Soviet Union gone, the SIS had asked him not to write a book and were "less than enthusiastic".

"Some things I have kept secret, like operational names and the identities of my colleagues.

"There are some operational things that even after 30 years must remain secret, but so much more of it should be in the public domain because it is a case that almost polarised the whole community.

"I never wrote it as an expose but as a ... fascinating part of New Zealand history."

Former New Zealand defence analyst Jim Rolfe, who lectures at the Asia-Pacific Centre for Security Studies in Hawaii, said the book would reinforce the views of people who believed Dr Sutch guilty, while others would see it as more persecution.

He said there would be some disquiet from the SIS that a retired officer had published a book, but he doubted if the service would do anything.

"They have been burned too often trying to stop secrets once they have been let out."