Eighteen years on from the murders of Ben Smart and Olivia Hope, the man convicted of killing them has finally broken his silence.

Scott Watson was granted approval to go on the record with North & South journalist Mike White in August, subject to certain conditions related to security of the prison and the interests of victims.

Watson is currently serving a life sentence with a minimum non-parole period of 17 years for the murders of 21-year-old Smart and 17-year-old Hope on January 1, 1998.

The pair disappeared after boarding a yacht with a stranger in the Marlborough Sounds and the case remains one of the most controversial in New Zealand's history.

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In three interviews conducted over three days from Christchurch's Rolleston Prison, Watson maintained his innocence.

Of his arrest, he said, "I think it was because I had a criminal record and I was [at Furneaux Lodge] alone and I left alone.

"Basically, I was an easy target from them," he told White, a former Marlborough Express reporter who covered the case from the beginning. "I was the easiest person that they could pick.

"I don't know where Ben and Olivia are," he said. "I've never met them, never seen them.

"They definitely never came on my boat and I definitely didn't murder them. And they've basically dumped me in jail for half my lifetime, it must be coming up, for something I haven't done.

"It's destroyed my family and my life."

In the interview, Watson also discussed the events of that fateful night, the days that followed and his realisation he was the prime suspect in the Smart and Hope murders, as well as crimes he committed as a teenager, his drug use and the "growing up" years.

"I got over it, and got on with life. You grow up, you get a job."

With what White describes as "unwavering honesty", Watson also discussed his experience of the Smart and Hope murder trial and his time in jail.

On whether he believes Watson is innocent, White says that's "a mug's game".

"You don't judge a person on how you perceive them. You judge them on what you know of the case and the evidence you can rely on.

"Into that, we can now add what Scott Watson has said. The only thing I can say is that nothing he told me in Rolleston Prison has eased my enormous disquiet about this case and the many, many flaws and holes in it."

However, White says, none of this is done to cause distress to the Smart and Hope families, or to "dredge up old cases".

"Our justice system isn't infallible and we must be as certain as we possibly can that the right person is convicted for the right crime."

Mike White's interview with Scott Watson, Scott Watson: The Interview, will be available in the December issue of North & South, on sale Monday, November 16.