A law professor has cast fresh doubt on the evidence used to convict Scott Watson of double murder in the Marlborough Sounds nearly 20 years ago.

Watson remains in jail convicted of one of New Zealand's most controversial murders after 17-year-old Olivia Hope and her boyfriend 21-year-old Ben Smart boarded a yacht in the Marlborough Sounds with a man in the early hours of New Year's Day 1998 and were never seen again.

Watson was convicted of killing the pair despite their bodies never being found.

He has steadfastly maintained he had no involvement in their disappearance .

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Last night a new documentary Doubt: The Scott Watson Case was aired on TVNZ 1.

The show's presenter, Massey University Law professor Chris Gallavin, challenged the grounds of Watson's conviction arguing the trial and investigation were unfair. This morning he told Breakfast there were elements in the trial that amounted to a miscarriage of justice.

"I don't know if the wrong man went to jail but I certainly know it was a great surprise that he was convicted both in my mind and also in many people's minds simply because of the paucity of evidence," Gallavin said.

He said evidence presented in the trial was based on "shaky grounds".

"There were 1600 people at Furneaux Lodge that night, not all of them were sober or behaving well, in fact there would have been a great many drunk males probably clowning around and acting the fool as he was and acting inappropriately as he was and it just seemed that they had a paucity of evidence at the time and the more I've looked into it and more New Zealand's looked into it - there's five books been written on this case of course - then we've all been left with lingering doubts as to whether Scott Watson was really the man."

Police reiterated their stance that evidence surrounding the Sounds double murder had been heard and tested in multiple courts.

Scott Watson in High Court Christchurch. Photo / Pool
Scott Watson in High Court Christchurch. Photo / Pool

"The evidence against Scott Watson has been heard and tested in several courts, including the Court of Appeal.

"The Operation Tam investigation has also been scrutinised by the Independent Police Conduct Authority.

"Mr Watson's conviction by a jury is based on consideration of all the evidence, not just individual pieces of the picture," police said in a statement shown at the end of the documentary.

This included witness testimony corroborated by the facts.

Watson's father, Chris Watson, said the documentary made "a few good points", and "got down into how the investigation went".

He said witnesses were "massaged" to get them to say what investigators wanted to hear.

"Now police will go into their little castle, pull their drawbridge up, throw a few more sharks in the moat," he said.

He said there was "so much wrong" with the investigation that it couldn't all be addressed "in one hit".

"Of course, the system doesn't want to listen, so that's the end of it."

Today, author Ian Wishart slammed the documentary as little more than an exercise in cinematography.

He said it reheated old material focusing on accounts of a mystery man that was different from Watson and an attempt to bamboozle the public with multiple ketch sightings.

Wishart, who has written three books on the Watson case, is tonight presenting a free online webinar on the double murder trial.

This morning, a social media group dedicated to freeing Watson has had a major boost in interest thanks to the documentary, doubling in size overnight.

A protest is due to take place next Tuesday at Parliament to generate public awareness about the case in a bid to overturn the murder conviction.