20 years on, the Sounds murders are back in the spotlight

Monday marks the 20th anniversary of one of New Zealand's most captivating and perplexing murder cases.

Olivia Hope and Ben Smart disappeared after a New Year's Eve party at Furneaux Lodge in the Marlborough Sounds.

Police accused Scott Watson of killing them on his yacht and he's still in prison for their murders.

But now the lawyer and investigator who helped clear Teina Pora's name are taking another look at Watson's conviction.

Watson's 70-year-old father, Chris, has always believed his son is innocent.

"I've never had any doubt about [his innocence] - and not just because Scott said so," he told Newshub.

Hope and Smart disappeared in the early hours of New Year's Day, 1998. They were last seen getting off a water taxi and onto a boat, which police would later say was Watson's home-built sloop, Blade.

Their bodies were never found, but Watson was convicted in May 1999 for the double-murder, with police linking two blonde hairs found on his boat to Olivia Hope.

Chris Watson has remained critical of the police investigation.

"There was a definite campaign of separation - to separate us from the community, as well as from each other," he said.

He's particularly scathing of police search efforts in the Sounds to find a double-masted ketch.

Some witnesses described seeing Smart and Hope get onto it with a scruffy mystery man, while several others reported seeing it leaving the Sounds in the days ahead.

"That boat could have gone 150 miles in any direction before anybody knew or cared," Chris Watson said.

He says leading detective Rob Pope got side-tracked with his fixation on his son and his single-masted sloop, Blade.

Mr Pope's colleague and fellow Watson detective John Rae wouldn't talk to Newshub, only saying: "It's been 20 years since it happened... and most of it's been said in the media before."

But in a recent NZME podcast, Mr Rae revealed that he has no doubt of Watson's guilt and believes Watson knows where the bodies are.

Olivia Hope's father Gerald also turned down Newshub's interview request and his last public comments on the case came last year after a face-to-face prison meeting with Watson.

Of that meeting, Mr Hope told Fairfax that Watson pitched to him that he was innocent, but he wasn't able to accept that he was, based on some of his omissions.

To this day, Watson continues to fight to clear his name.

Recently he enlisted lawyer Jonathan Krebs and investigator Tim McKinnell - the pair known for helping free Teina Pora, who was wrongly imprisoned for 20 years for the rape and murder of Susan Burdett.

Mr Krebs said Watson's convictions deserve "another look".

"He's very grateful to his supporters... who have taken the time to commission a fresh scientific report - and that's all I'm going to say about what's in the fresh evidence," he said.

Because Watson has exhausted all standard avenues of appeal, Mr Krebs has filed an application for a Royal Prerogative of Mercy.

It's now in the hands of the Governor General, who takes advice from the Ministry of Justice, meaning Justice Minister Andrew Little will be influential in any decision on whether to grant Scott Watson a second appeal 20 years after the murders.