What is the test for reviewing a decision-making power of a body in the context of constitutional review analysis?

In Dunsmuir, the majority addressed Crevier, stating at para. 29: Administrative powers are exercised by decision makers according to statutory regimes that are themselves confined. A decision maker may not exercise authority not specifically assigned to him or her. By acting in the absence of legal authority, the decision maker transgresses the principle of the rule of law. Thus, when a reviewing court considers the scope of a decision-making power or the jurisdiction conferred by a statute, the standard of review analysis strives to determine what authority was intended to be given to the body in relation to the subject matter. This is done within the context of the courts’ constitutional duty to ensure that public authorities do not overreach their lawful powers: Crevier v. Attorney General of Quebec, 1981 CanLII 30 (SCC), [1981] 2 S.C.R. 220, at p. 234; also Dr. Q v. College of Physicians and Surgeons of British Columbia, [2003] 1 S.C.R. 226, 2003 SCC 19, at para. 21.