A new report into witness reliability in Scott Watson's second attempt before the Court of Appeal has been challenged by the Crown.

Watson was found guilty of murdering friends Ben Smart and Olivia Hope in 1999.

The pair were last seen getting off a water taxi onto a yacht in the Marlborough Sounds on New Year's Eve.

Open Justice reporter Catherine Hutton says the report presented as evidence by the defence today argues many witness accounts were unreliable in his trial.

Meanwhile the Crown responded, saying there were many holes in the contents of the report.

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