What is the test for "discretionary discretion" in the context of a regulatory body's interpretation of legislation?

In Roncarelli v. Duplessis, 1959 CanLII 50 (SCC), [1959] S.C.R. 121, 140, Rand J. said: In public regulation of this sort there is no such thing as absolute and untrammelled “discretion”, that is that action can be taken on any ground or for any reason that can be suggested to the mind of the administrator; no legislative Act can, without express language, be taken to contemplate an unlimited arbitrary power exercisable for any purpose, however capricious or irrelevant, regardless of the nature or purpose of the statute. …. “Discretion” necessarily implies good faith in discharging public duty; there is always a perspective within which a statute is intended to operate;