The retired detective who put the "Beast of Blenheim" behind bars in the mid-1990s says the rapist is likely to have victims who have never come forward, and whose cases could be grounds for a fresh trial.
Stewart Murray Wilson, 65, will be paroled in September after serving 18 years of a 21-year sentence for offences spread over 25 years against 16 women and girls, including rape, child cruelty and bestiality.
Wilson is considered especially high risk because he has refused to acknowledge his offending or undergo any therapy.
A new law allowing detention of high-risk sex offenders beyond the end of their sentences is being drafted for Justice Minister Judith Collins, but won't be in place in time to prevent Wilson's release.
But former Blenheim police detective Colin McKay says Wilson was such as prolific offender that he believes there are other victims who chose not to report the crimes.
Those victims could make a new trial, and further detention of Wilson, possible.
Mr McKay said that during Wilson's 1996 trial, numerous women came forward alleging he had committed offences against them, including rape.
These "late-starter" allegations were not investigated or taken to trial because police already had a solid case. Instead they were added to the police's file on Wilson, which eventually filled a cabinet "six feet high".
Now, as concerns rise about the risk Wilson poses once released, Mr McKay says a fresh complaint could be enough to trigger a fresh investigation.
"If someone walked into the police station in Blenheim today and said they were sexually abused 50 years ago, the police would investigate it and take whatever action was necessary."
Police told Fairfax they had "no plans to revisit the investigation as a means of seeking information for additional charges", because to do so "might amount to an abuse of process".
But if a fresh complaint was laid, they would evaluate the information, investigate it if warranted, and lay charges if appropriate.
Crown Solicitor's Office prosecutor Jackson Webber said a new prosecution was not out of the question.
While he had no specific knowledge of Wilson's case, Mr Webber said there was no statute of limitations, "so if there were complainants who wished to take the matter forward at this point, it is possible that charges could be laid".
Trying historical crimes was always tough, said Webber, because people moved, crime scenes changed and memories faded. A complainant would also have to be willing to go to trial, because without a willing witness, police would have no case.
Otago law professor Geoff Hall said if police simply hauled out the uninvestigated complaints that came out of the woodwork during the 1996 trial and used them as the basis for new charges, Wilson could complain about the delay, and block a new trial. But if a new complaint was laid, there might be grounds for a prosecution. Fairfax NZ
- The Marlborough Express
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