Ben Smart and Olivia Hope. Photo / Supplied

Ben Smart and Olivia Hope. Photo / Supplied

Convicted killer Scott Watson's bid for freedom has been boosted by the appointment of a top taxpayer-funded lawyer to reinterview key witnesses in the case of Ben Smart and Olivia Hope.

The Ministry of Justice has hired Kirsty McDonald QC to review Watson's latest legal move, and she identified key areas of "potentially fresh evidence" during those interviews.

Journalist Keith Hunter, who has led the campaign to free Watson, is planning a formal interview of another witness, water taxi operator Guy Wallace, this week.

Watson has always maintained he is not guilty of murdering Hope and Smart, who disappeared in the Marlborough Sounds on New Year's Day in 1998.

He was found guilty of murdering both, largely on evidence which has since been questioned.

The developments follow Watson's last-ditch move for another chance at court - or freedom - by applying to the Governor-General for a Royal Prerogative of Mercy.

The application has three possible outcomes - denial, a full pardon, or a hearing in the Court of Appeal.

The application is investigated by the Ministry of Justice which will advise Justice Minister Simon Power of its findings.

Power will pass those findings to the Governor-General for the final declaration.

The ministry appointed McDonald for legal analysis and inquiry work.

A letter from Power to Watson - supplied to the Herald on Sunday through Watson's dad Chris - said McDonald had reviewed the application and found possible evidence unavailable at the time of the trial. Power said Watson had to provide evidence to support the areas that had been identified but added that some of the material would be gathered by McDonald.

Among the evidence was an interview with Secret Witness A, whose identity was protected when he gave evidence that Watson had confessed to the killings while the pair were in prison together.

The man later recanted the testimony in a taped interview with the New Zealand Herald, saying he had been pushed into it by police.

Power considered it "more preferable for a Queen's Counsel to endeavour to interview Secret Witness A and assess the reliability of his evidence".

It followed advice from McDonald that Secret Witness A should be quizzed and "formal evidence" taken on his retraction.