A former police detective who worked on the Scott Watson double murder case says public tips offered to investigators need to be revisited.
Watson was found guilty of killing Ben Smart and Olivia Hope, who disappeared in the Marlborough Sounds on New Year's Day in 1998.
Watson has now applied for a Royal Prerogative of Mercy from the Governor-General.
Justice Minister Simon Powell says Kristy McDonald, QC, has been hired by the ministry as part of the application.
Mr Powell says she has identified seven points of potential fresh evidence, which have been referred back to Watson and his team for further advice.
These include trial evidence that has since been retracted, new evidence from key witnesses and a greater scrutiny of sightings of yachts matching the description of a two-masted ketch.
Former detective Michael Chappell says the police investigation was remarkably quick to discount a ketch as a possible location for the killing.
A number of people identified a two-masted ketch in the area and their calls should be reviewed, he says.