The latest appeal on behalf of convicted double murderer Scott Watson lies in the hands of an unexpected supporter.

Watson is in prison for the murders of Olivia Hope and Ben Smart, who disappeared on New Year's Day 1998.

A donor-funded forensic report is being used as part of an application for a Royal Prerogative of Mercy.

It challenges whether two blond hairs found on Watson's boat were actually those of Olivia Hope.

Brian McDonald is leading the appeal.

Having spent 15 years behind bars himself for murder and manslaughter, he said he would hate to see an innocent man suffer.

''I'm certainly not interested in helping a guy escape lawful convictions. In my mind, there is little doubt that he is not guilty.''