Duty to the court.
Part 33.2 of The Criminal Procedure Rules 2010 No. 60 (L.2) says...
(1). An expert must help the court to achieve the overriding objective by giving objective, unbiased opinion on matters within his expertise.
(2). This duty overrides any obligation to the person from whom he receives instructions or by whom he is paid.
(3). This duty includes an obligation to inform all parties and the court if the expert’s opinion changes from that contained in a report served
as evidence or given in a statement.
Also see an expert's duty to the court in civil cases.