In reaction to Ian Wishart's new book and retraction of his earlier belief that Scott Watson is innocent, the Police Association says the "continual issuing of books and other publications casting aspersions on verdicts by honing in on one aspect of a case and ignoring other evidence is damaging to the public's faith in the justice system".

The association's president, Greg O'Connor said the essence of detective work was to never allow the evidence to fit the theory of what occurred.

"Every new piece of evidence should be interrogated and its impact on the reconstruction of the crime evaluated."

Mr O'Connor said the "reality" of a police inquiry was that all known facts were not only gathered but were "evaluated, corroborated where possible and, importantly, tested for their impact on the reconstruction of what occurred. The essence of good detective work is to continually reconstruct until the evidence is exhausted."

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"The continual issuing of books and other publications casting aspersions on verdicts by honing in on one aspect of a case and ignoring other evidence is damaging to the public's faith in the justice system. This is often what individuals accuse police of doing.

"The system is not perfect, and mistakes can be made. However, the checks and balances on police in the modern era, through peer review, Crown oversight, the judicial system (including the appeal process) and the Independent Police Conduct Authority, act as a filter which the plethora of post-verdict books are not subject to."

Mr O'Connor said New Zealanders to view any such publications with "a healthy dose of scepticism".

"They are generally only the views of an individual whose theories on what are often narrow aspects of a case are not subject to any of the scrutiny applied to the Police and Crown cases."