Nothing has changed the minds of the senior police officers about Watson's guilt, although Pope - who went on to become the deputy police commissioner - diplomatically acknowledges high-profile cases will always attract criticism.

"To be quite frank I wouldn't have it any other way," says Pope. "It's one of the great values living in a democratic country like New Zealand and having a judicial system which is sound, fair and transparent."

Rae, who retired several years ago, is more blunt.

"There are a few champions for the cause of Scott Watson for which I have no time. In actual fact, I have no doubt in my mind that he is guilty.

"I'm sure he knows where the bodies are."

They dismiss the "tunnel vision" criticism of the investigation to say there were 119 suspects - only Watson could not be eliminated.

And the report of Kristy McDonald, QC, which declined Watson's first bid for a royal pardon found most of the issues raised in were not "fresh". They had been raised at the trial where the jury convicted him.

Pope says the 12 jurors sat through 11 weeks of the trial and thousands of pages of evidence.

"That provides the most balanced and fair presentation of what actually the prosecution case was about," says Pope.

"It's very easy to read a book or a series of books that may focus on a particular aspect but a circumstantial case relies on consideration of the totality, not just the elements or snippets of it.

"There is no such thing as a 100 per cent watertight case. If there was I would be looking very suspiciously at that."

But there is regret: the bodies of Ben and Olivia were never found.

Scott Watson has been in prison since his conviction in 1999 and cannot apply for parole until 2020. (Photo / John Kirk-Anderson)

"To me that was our biggest failure," Rae says.

Pope: "My whole team would dearly have loved to bring home Ben and Olivia so that John and Mary [the Smarts], Jan and Gerald [the Hopes], could actually properly grieve.

"We all dearly would have loved to have brought finality."

Chris Watson steadfastly maintains his son Scott is innocent and the police cherry-picked the evidence to build the case around him.

"I know he didn't [kill them]," says Chris Watson. "And not from anything he's told me. I've looked at it and used my intelligence and commonsense."

He's kept Scott's boat, Blade, for his son to sail on his eventual release from prison. His next parole hearing is in 2020.

However, Chris Watson may not be alive to see him on the water again.

Scott Watson's father Chris is fighting cancer but will keep campaigning for his innocence. (Photo / Mike Scott)

He's been diagnosed with prostate cancer, but says the campaign to free his son will carry on.

"People are interested in this case. There are a lot of questions which need answering," Chris Watson said.

"I have got friends and family around the place who are offended by this sense of injustice - it will carry on after I am gone."