A tribunal of fact cannot insulate its fact-finding by asserting that it is all based on credibility. This obvious proposition was explained in Fox v Percy (2003) 214 CLR 118:
[28] Over more than a century, this Court, and courts like it, have given instruction on how to resolve the dichotomy between the foregoing appellate obligations and appellate restraint. From time to time, by reference to considerations particular to each case, different emphasis appears in such reasons. However, the mere fact that a trial judge necessarily reached a conclusion favouring the witnesses of one party over those of another does not, and cannot, prevent the performance by a court of appeal of the functions imposed on it by statute. In particular cases incontrovertible facts or uncontested testimony will demonstrate that the trial judge’s conclusions are erroneous, even when they appear to be, or are stated to be, based on credibility findings.
[29] That this is so is demonstrated in several recent decisions of this Court. In some, quite rare, cases, although the facts fall short of being “incontrovertible”, an appellate conclusion may be reached that the decision at trial is “glaringly improbable” or “contrary to compelling inferences” in the case. In such circumstances, the appellate court is not relieved of its statutory functions by the fact that the trial judge has, expressly or implicitly, reached a conclusion influenced by an opinion concerning the credibility of witnesses. In such a case, making all due allowances for the advantages available to the trial judge, the appellate court must “not shrink from giving effect to” its own conclusion. Finality in litigation is highly desirable. Litigation beyond a trial is costly and usually upsetting. But in every appeal by way of rehearing, a judgment of the appellate court is required both on the facts and the law. It is not forbidden (nor in the face of the statutory requirement could it be) by ritual incantation about witness credibility, nor by judicial reference to the desirability of finality in litigation or reminders of the general advantages of the trial over the appellate process.
(footnotes omitted)