What is the test for judicial deference to an administrative tribunal on the interpretation of its "home statute" or statute?

It may be recalled that the willingness of the courts to defer to administrative tribunals on questions of the interpretation of their “home statutes” originated in the context of elaborate statutory schemes such as labour relations legislation. In such cases, the tribunal members were not only better versed in the practicalities of how the scheme could and did operate, but in many cases, the legislature tried to curb the enthusiasm of the courts to intervene by inserting explicit privative clauses. Over the years, acceptance of judicial deference grew even on questions of law (see, e.g., Pezim v. British Columbia (Superintendent of Brokers), 1994 CanLII 103 (SCC), [1994] 2 S.C.R. 557), but never to the point of presuming, as Rothstein J. does, that whenever the tribunal is interpreting its “home statute” or statutes, it is entitled to deference. It is not enough, it seems to me, to say that the tribunal has selected one from a number of interpretations of a particular provision that the provisions can reasonably bear, no matter how fundamentally the tribunal’s legal opinion affects the rights of the parties who appear before it. On issues of procedural fairness or natural justice, for example, the courts should not defer to a tribunal’s view of the extent to which its “home statute” permits it to proceed in what the courts conclude is an unfair manner.