Gerald Hope has said his last word in a case that's captivated the nation for nearly 19 years.

A month after a face-to-face meeting with convicted killer Scott Watson, Mr Hope said he'll do no more interviews regarding the disappearance of his daughter Olivia and Ben Smart on New Year's Day, 1998.

After talking with Watson for nearly six hours over two meetings, he walked away unconvinced of Watson's innocence.

Mr Hope said when asked about evidence like the sighting at Marine Head and his arrival time at Erie Bay, Watson seemed mute, unemotional and disconnected.

Gerald Hope's daughter, Olivia Hope, was 17 when she and her friend Ben Smart, 21, disappeared on New Year's Day in 1998. They were last seen in the Marlborough Sounds boarding a yacht in with a man early that morning.

Their bodies were never found. Picton man Scott Watson was charged with and convicted of both their murders. He was sentenced to life in prison.

In 2007, Hope told the Herald he had growing doubts about whether Watson was guilty and would help fight to free the convicted killer if he was convinced of Watson's innocence.

"What we got was a conviction but we never got the truth. And that's the part that still really rips me up. Nothing ever was confirmed, it was all circumstantial, there was no hard evidence. And that's where my greatest doubts lie," Hope said at the time.

A decision on Scott Watson's latest bid for parole is expected before Christmas.