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 pair went missing after New Year's celebrations at Endeavour Inlet and their bodies have never been found.
The IPCA yesterday released its findings on complaints about the police investigation, named Operation Tam, from journalist Keith Hunter and Watson's father Chris.
Authority chairwoman Justice Lowell Goddard said that apart from three aspects "that had no significant bearing on the outcome of the investigation, Operation Tam was conducted reasonably and rationally and the police officers who led it remained open-minded throughout".
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.
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 authority found no evidence to support Mr Hunter's allegation that DNA evidence -- Olivia Hope's hair -- was either "planted" on a blanket from Watson's boat Blade, or poor police practices resulted in accidental cross-contamination.
The IPCA similarly found no evidence to support the allegation that Mr Pope gave orders to cease searching for a "mystery ketch", despite receiving numerous eyewitness accounts of it. The authority said police went to considerable lengths to identify, locate and eliminate all vessels in the vicinity of Furneaux Lodge where Miss Hope and Mr Smart had been celebrating the New Year.
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.
