A new book saying Scott Watson was guilty of the murder of Ben Smart and Olivia Hope is just a conspiracy theory, Watson's father says.
Investigative journalist Ian Wishart has changed his view and now believes Scott Watson is guilty of murdering the pair, but had an accomplice.
Olivia Hope, 17, and Ben Smart, 21, were last seen in the early hours of 1 January 1998. Scott Watson is serving a life sentence, with 19 years minimum, in prison for their murders.
Mr Wishart has published a book, Elementary, on the deaths which has been released this morning.
Mr Wishart told Morning Report there were two men on board Watson's boat Blade within hours of Ben and Olivia going missing.
He said he had police papers containing witness statements from people on three separate boats - including that of an eight-year-old boy - saying another man was on the Blade.
"I went for three witness sightings and we had one of Scott Watson's friends confirming that he saw Scott Watson on the boat with another person."
"Therefore there can be no doubt about the fact the eight year old was right, and he's just one of three separate boats that saw Scott Watson at that same place in same time painting his boat"
"I am deeply saddened, 17 years after co-writing the first book on this case questioning the guilty verdict, to find out that Watson was indeed the killer of Ben and Olivia. It has not been easy to essentially retract the findings of that book, but it is the right thing to do," he said in a statment on the release of the book.
And Mr Wishart has called on Watson to admit he murdered the pair and to tell investigators where the bodies of the pair are buried.
Mr Wishart said there was evidence in the police file that suggested the bodies had been buried on land and therefore there was a chance they could be recovered.
"The best thing in my view is for Scott to simply say 'I did this, I'm sorry, this is where the bodies are.' Because the families want some closure."
Watson's father Chris Watson had still to read the book, but said the witness statements did not mean anything.
"A number of people have read them, have had them, and nobody else has come to that conclusion," he told Morning Report.
He had no doubt about his son's innocence.
"It's a conspiracy theory now, isn't it," he said of the book.
He rejected the assertion that any appeal attempt would not stand up. "Appeal attempts don't stand up because the system won't allow them, they only work on points of law."
Watson has continued to plead his innocence. Last year he had his first parole hearing after 17 years of his life sentence for the murders, but he was turned down, with the Parole Board saying he still posed an unacceptable risk to the public.
He did win a legal fight to speak to a journalist for the first time since his conviction, and told North and South reporter Mike White that he was still angry about the way he had been treated.
The case has already been in the headlines again this week, with a previously unreleased photo supposedly showing Ms Hope days after her murder published by the NZ Herald.
The photo, released by a group calling itself the Maritime Research Group, was part of a new theory about how the couple met their end, claiming a drug syndicate was responsible.
Speaking to Checkpoint with John Campbell yesterday, Olivia Hope's father Gerald Hope said the photo was just too grainy and he could not confirm that it was his daughter.