Police fell short in some areas of their investigation into the murders of Ben Smart and Olivia Hope, but their mistakes did not have a significant bearing on the inquiry's outcome, the Independent Police Conduct Authority (IPCA) has found.
Scott Watson was sentenced to life imprisonment with a minimum non-parole period of 17 years after being found guilty of the 1997 murders in the Marlborough Sounds.
The IPCA launched an inquiry into the police investigation after it received complaints from journalist Keith Hunter and Watson's father Chris.
No complaint was received from Scott Watson himself.
In a report released today, the IPCA found the investigation, known as Operation Tam, fell short in three respects.
An affidavit to obtain search warrants signed by Detective Inspector Rob Pope, now the Deputy Commissioner, contained errors and fell short of the high standard of accuracy needed for warrants.
However, the authority rejected Mr Hunter's allegation that Mr Pope had intended to mislead, and found the errors did not "strike at the heart of the document".
Mr Pope had not known the document was inaccurate when he signed it, and his mistake did not constitute misconduct or neglect of duty.
The authority found it was "highly undesirable" for a member of the investigation team to give a suspect profile of Scott Watson to a community group.
It also found the construction of photo montages of Watson, and the methods used to show them to witnesses, were undesirable.
However, a lack of documentation meant the authority could not determine how decisions about the montages were made, and by whom.
Mr Hunter had complained the montages put Watson at a disadvantage because they showed him with his eyes half-closed.
Authority chairwoman Justice Lowell Goddard said today that areas in which Operation Tam fell short had no significant bearing on the outcome of the investigation
The inquiry was conducted reasonably and rationally, and police remained open-minded throughout.
"On the basis of what Detective Inspector Rob Pope knew, it was reasonable and appropriate for him to focus resources on investigating Scott Watson. Indeed, he would have been remiss had he not," Justice Goddard said.
The police investigation was conducted in a small community in which many people knew each other and discussed the case openly.
It also took place under intense media scrutiny, with witnesses and people who knew Scott Watson frequently appearing in the media.
"The unprecedented and unrelenting nature of the media focus throughout the inquiry can only have served to make an already complex and difficult investigation even more so," Justice Goddard said.
"In these difficult circumstances, some actions of police fell short of best practice, some mistakes were made, and these were compounded by the actions of others, in particular the media and members of the community who openly discussed the investigation with each other and with reporters."
The IPCA found no evidence behind many of Mr Hunter's complaints.
Mr Hunter alleged Mr Pope had concluded Watson was guilty within five days of the investigation, ignored evidence to the contrary, created and circulated false rumours about Watson, told a "strategic lie" to media about whether Watson was a suspect, bought the testimony of secret witnesses, and planted DNA evidence.
The authority noted that many of the issues raised in Mr Hunter's complaint were available to Watson's defence team to raise at trial and on appeal.
However, it noted also that the conduct of the trial and appeal were not matters within its jurisdiction.
Justice Goddard said the authority's investigation was exhaustive, with investigative staff and legal analysts spending hundreds of hours examining some 25,000 documents.
Operation Tam was the largest police investigation ever conducted by New Zealand police at the time. About 1650 people who were in the Marlborough Sounds at the time had to be contacted and more than 100 vessels were identified and traced.
NZPA