What is the test of patent unreasonableness?

Patent unreasonableness is a strict standard. As pointed out by Cory J. in Canada (Attorney General) v. Public Service Association of Canada, 1993 CanLII 125 (SCC), [1993] 1 S.C.R. 941 at 963-64, the decision of an administrative body will be immune from review under a patently unreasonable standard, where it “is not clearly irrational, that is to say evidently not in accordance with reason...” The test of patent unreasonableness is principally a jurisdictional test. Iacobucci J. sheds some light on the application of the standard in Canada v. Southam Inc., supra at 777: The difference between “unreasonable” and “patently unreasonable” lies in the immediacy or obviousness of the defect. If the defect is apparent on the face of the tribunal’s reasons, then the tribunal’s decision is patently unreasonable. But if it takes some significant searching or testing to find the defect, then the decision is unreasonable but not patently unreasonable....This is not to say, of course, that judges reviewing a decision on the standard of patent unreasonableness may not examine the record. If the decision under review is sufficiently difficult, then perhaps a great deal of reading and thinking will be required before the judge will be able to grasp the dimensions of the problem. But once the lines of the problem have come into focus, if the decision is patently unreasonable, then the unreasonableness will be evident. [citations omitted]