The key police witness in the Scott Watson case believes he knows the true identity of the "mystery man" last seen with Ben Smart and Olivia Hope.

Guy Wallace was serving drinks at Furneaux Lodge on New Year's Eve 1997 and later drove a water taxi to ferry passengers to their boats after the party wound down.

He was one of the last people to see Ben Smart, 21, and Olivia Hope, 17, alive after dropping them at a boat with a "mystery man".

This man, whom Wallace had served drinks at the bar earlier in the night, was described as "sleazy", with unkempt, wavy hair and facial hair.

Photographs of Watson from the night show he was clean-shaven, with short hair.

But by April, four months later after the pair disappeared, no one aboard the water taxi, the last people to see Ben and Olivia alive, had identified Watson as the mystery man.

Guy Wallace was a key witness in Scott Watson's conviction. (Photo / NZ Herald)

Wallace, the crucial witness, had been shown Watson's photo at least three times.

Each time, he said Watson was not the mystery man - whom he had also served drinks at Furneaux Lodge, along with bar manager Roz McNeilly.

Police then showed Wallace a new image of Watson in a montage of eight photographs.

In this new photo, Watson was caught half-way through a blink. This gave the appearance of hooded eyes, a characteristic of the mystery man's description.

Wallace picked Watson from the "blink" photograph as the single man on the water taxi.

This gave Watson the impression of hooded eyes, matching the description of the mystery man.

Wallace and McNeilly have since recanted their evidence. He is also adamant he dropped Ben and Olivia at a ketch - much larger than Watson's Blade.

In an interview for the Herald's podcast series Chasing Ghosts: Murder in the Sounds, to mark 20 years since Ben and Olivia disappeared, Wallace says he feels guilty about dropping them off with the "mystery man".

He also feels guilty about identifying Watson and believes he knows who the "mystery man" really is - someone with connections to the criminal underworld and is already behind bars.

" These people need their kids laid to rest," is Wallace's message to the mystery man.

"If you have done something with them, be a man and grow a set."