The Independent Police Conduct Authority has dismissed complaints laid over the investigation into the disappearance and murders of Ben Smart and Olivia Hope.
The pair went missing from from Endeavour Inlet in the Marlborough Sounds on New Year's Eve in 1997.
Scott Watson was later convicted of their murders and jailed. Their bodies have never been found.
Journalist Keith Hunter and Watson's father, Chris Watson, had laid complaints over the investigation.
They included that the head of the inquiry, Inspector Rob Pope, deliberately ignored relevant evidence, spread rumours about Scott Watson and swore false affidavits to obtain search warrants.
The complaints also alleged that police bought the testimony of secret witnesses and deliberately or accidentally contaminated DNA evidence.
But IPCA chair Justice Lowell Goddard says Operation Tam was conducted reasonably and rationally, and officers remained open-minded throughout.
Justice Goddard says the case was subject to unprecedented media scrutiny and though some actions of police were short of best practice and mistakes were made, many of the issues were known to Watson's defence team during his trial and appeal.
The judge says the IPCA's review of the investigation was exhaustive, involving many hundreds of hours of work by investigative staff and legal analysts who examined 25,000 documents.
