Scott Watson supporters have launched an 0800 number for information on the high-profile double murder of which he was convicted almost 20 years ago.

Watson has been behind bars since he was convicted in 1999 of killing Ben Smart and Olivia Hope, who disappeared after boarding a yacht in the Marlborough Sounds early on New Year's Day 1998.

Police said the yacht was Watson's sloop, others claim it was a mystery ketch and Watson is innocent.

Free Scott Watson Campaign member Aaron Goodwin said the hotline - 0800 379 111 - was set up to allow the public a chance to come forward with information on the disappearance of Smart and Hope.

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"Scott Watson supporters have always maintained that there are people out there who have information and haven't come forward. This was confirmed over the weekend as members of the public conversed with Free Scott Watson campaigners around the upper South Island during a protest that covered seven different towns and cities."

Watson's family and legal team will be the first told of any relevant information, Goodwin said.

The hotline goes live within the next 24 hours, he said.