The issue of expert evidence, given on the basis that the expert only be paid should the party retaining him or her succeed, has been the subject of some debate. The Australian Medical Association appears uncomfortable with any such arrangements, stating in the current Ethical Guidelines for Doctors Acting as Medical Witnesses – 2011:
5.7 It is important for you to remain independent and remote from the litigation. Your role is not to plead the merits of the case for the side paying your fee. You are not an advocate. You should have no interest in the outcome of the case, and you should be seen to have no interest in the outcome of the case.
An interlocutory decision in Fuller-Lyons v State of New South Wales (No 2)  NSWSC 445 touched on the conditional fee issue regarding a (non medical) expert witness. It appeared that the expert may have been retained on a conditional fee basis, and had failed to disclose that to the court as he was unaware of the obligation to do so under the New South Wales Uniform Civil Procedure Rules 31.22. The defendant to the litigation sought to have the reports and oral evidence of the witness excluded.
In the circumstances of the matter, the trial judge did not exclude the evidence. The expert was unaware of the rule, did not seek to hide the basis on which he was retained and it was not suggested to him that the opinions expressed in his report were not bona fide held by him.