The prosecution of a previously respected public servant, Dr William Sutch, for spying, ended unsatisfactorily in 1975. Sutch was acquitted for lack of evidence that he had passed anything of value to a Soviet Union agent. But the acquittal did not explain the shadowy meetings he had been observed having with an officer of the Soviet Embassy on Wellington street corners at night over many months.

This week a book written by a former member of the Security Intelligence Service reveals a great deal about how the SIS went about the Sutch investigation. Kit Bennetts' account has been produced against the wishes of the service, but we have published excerpts over the past two days because the Sutch case remains an important experience for New Zealand long after the Cold War has been consigned to history.

The book provides rare and valuable insights to the operations of the SIS, as well as descriptions of Sutch's behaviour that will cause many to wonder why he did it. Why did a man who had been highly respected and moved in the most rarefied government circles stoop to having clandestine meetings with a Soviet agent on cold street corners in the dead of night? The question raises conjecture not just about what sort of information Sutch may have passed but also about the nature of loyalty and treachery, questions that have enduring interest today.

The events Bennetts describes occurred at a time that New Zealand was strongly aligned to the West in the global power struggle with Soviet communism. Labour and National were committed to the Anzus Alliance and, whichever was in power, New Zealand was a trusted member of the West, privy, perhaps, to much of its strategic intelligence. As the smallest and most distant of the allied capitals it is possible that Wellington would be seen by the Soviets as a place that would have more difficulty keeping secrets secure.

Then, as now, there were many New Zealanders who did not take this country's strategic position particularly seriously. And then, unlike now, some well-placed people in most Western countries were unconvinced history was on their side. The Marxist vision, in which the world was in transition from selfish capitalism to an ultimately stateless communism, retained a powerful attraction to some older educated people in the West long after the realities of Soviet life were well known.

It is not known whether Sutch's left-wing views went so far, it is possible that simple self-importance led him to think he should keep the Soviets informed of whatever he thought proper to share with them. Whatever it was, Bennetts is probably right to say of Sutch, "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 ... his belief that he perhaps knew better."

It is encouraging to find this sort of thinking in someone trained in counter-intelligence. Spy catchers need to know the psychology of those who might betray national interests. They will not exhibit the treacherous intent of fiction's villains, they will more likely be highly principled people whose sympathies do not wholly accord with New Zealand's official stance and who are sufficiently conceited to be thought capable of undermining it.

Watching people, following them, ferreting out their private proclivities, is work that most readers of the extracts will find deeply distasteful but the Sutch case remains the single known example of the necessity for counter-intelligence in this country. Bennetts' unauthorised account has done the SIS a service.