"[T]he Tribunal [has a] duty to act judicially in the present case. That being so, it was necessary that any findings of fact made by the Tribunal, upon which a reviewable 'decision' was based, were supported by some probative material which was properly before the Tribunal. If a finding of fact was not so supported, a 'decision' which was based upon it was invalid. In that regard, it would matter not that the decision could be supported by some other finding of fact which was open to the Tribunal but which the Tribunal had not made. The point can be illustrated by reference to a hypothetical case where a decision could be supported by either a finding of fact A or a finding of fact B and where there was probative material to support a finding of A but no probative material at all to support a finding of B. If, in such a case, the Tribunal stated that it made no finding about, and placed no reliance upon, A but based a reviewable 'decision' on a positive finding of B, the Tribunal would have failed to discharge its duty to act judicially. Its decision would be based on a finding of fact which, being unsupported by any probative material, was, as a matter of law, not open. It would simply be irrelevant to say that there was probative evidence upon which the Tribunal had not relied which would have supported a finding of A which the Tribunal had neither made nor relied upon. Therein, to my mind, lies the compelling force of [the requirement] that a decision made in compliance with [the requirements of natural justice and procedural fairness] must be 'based' upon probative and relevant material (see, also, Mahon v Air New Zealand)."[57]