From the decision:

Justice Gascon wrote as follows (at paras. 6, 30, 32-34, 38, 74-75):

"I would dismiss the appeal. The principle that the motives of a legislative body are “unknowable” and deliberative secrecy do not apply to a public employer, the Board in this case, that decides to take disciplinary action against an employee, even if an in camera meeting is ordered. The three members of the Board’s executive committee can be examined, subject to the limits of what is relevant and to the other rules applicable to the inquiry into the grievance. The arbitrator has exclusive jurisdiction to determine whether any questions that may be asked are relevant.

…

Unlike the judges of the Court of Appeal and the Superior Court, I find that the standard applicable to the arbitrator’s interlocutory decision is reasonableness. Whether the examination of the members of the Board’s executive committee should be allowed is ultimately an evidentiary issue. The arbitrator has exclusive jurisdiction over such matters. In my opinion, a desire, like that of the appellants, to attribute an excessive scope to this Court’s decisions in Clearwater and Tremblay does not transform this determination into a question of law that is of central importance to the legal system and is outside the arbitrator’s area of expertise, such that the standard of correctness should apply.

…

In Alberta (Information and Privacy Commissioner) v. Alberta Teachers’ Association, 2011 SCC 61, [2011] 3 S.C.R. 654, the Court stated that when an administrative tribunal interprets or applies its home statute, there is a presumption that the standard of review applicable to its decision is reasonableness (paras. 39 and 41; see also Tervita Corp. v. Canada (Commissioner of Competition), 2015 SCC 3, [2015] 1 S.C.R. 161, at para. 35; Smith v. Alliance Pipeline Ltd., 2011 SCC 7, [2011] 1 S.C.R. 160, at paras. 26 et 28; Dunsmuir, at para. 54). That presumption applies in the case at bar. The arbitrator’s decision to allow the Union to examine the executive committee’s members was based on his conclusion that their testimony would be helpful to him in determining whether the collective agreement and the legislation had been complied with. This conclusion flowed from his interpretation of the local agreement between the parties and of the EA. His home statute, the Labour Code, provides that an arbitrator may “interpret and apply any Act or regulation to the extent necessary to settle a grievance” (s. 100.12(a)). The Court has held that a reviewing court owes the greatest possible deference to an interpretation of provisions of the EA by a grievance arbitrator in an educational setting: Syndicat de l’enseignement du Grand‑Portage v. Morency, 2000 SCC 62, [2000] 2 S.C.R. 913, at para. 1.

The presumption is reinforced by the fact that the Court has held that the usual standard for judicial review of decisions of grievance arbitrators is reasonableness: Communications, Energy and Paperworkers Union of Canada, Local 30 v. Irving Pulp & Paper, Ltd., 2013 SCC 34, [2013] 2 S.C.R. 458, at para. 7; Newfoundland and Labrador Nurses’ Union v. Newfoundland and Labrador (Treasury Board), 2011 SCC 62, [2011] 3 S.C.R. 708, at para. 8; Dunsmuir, at para. 68. The Court added in Nor‑Man Regional Health Authority Inc. v. Manitoba Association of Health Care Professionals, 2011 SCC 59, [2011] 3 S.C.R. 616, that this standard is equally appropriate where the arbitrator applies or adapts, for example, common law and equitable doctrines that emanate from the courts: paras. 5‑6, 31 and 44‑45. This is because the grievance arbitrator is part of a discrete and special administrative scheme under which the decision maker has specialized expertise. In Quebec, moreover, the grievance arbitrator is protected by general full privative clauses (ss. 139, 139.1 and 140 L.C.; United Food and Commercial Workers, Local 503 v. Wal‑Mart Canada Corp., 2014 SCC 45, [2014] 2 S.C.R. 323, at para. 89).

The presumption from Alberta Teachers has not been rebutted in the instant case. The issues in this case are not included in the narrow class of issues identified in Dunsmuir for which the applicable standard is correctness. As the Court explained in Dunsmuir, that standard can apply to questions of law that are of central importance to the legal system as a whole and are outside the decision maker’s area of expertise (paras. 55 and 60). Such questions must sometimes be dealt with uniformly by courts and administrative tribunals “[b]ecause of their impact on the administration of justice as a whole” (para. 60). However, questions of this nature are rare and tend to be limited to situations that are detrimental to “consistency in the fundamental legal order of our country” (Canada (Canadian Human Rights Commission) v. Canada (Attorney General), 2011 SCC 53, [2011] 3 S.C.R. 471 (“Mowat”), at para. 22; McLean v. British Columbia (Securities Commission), 2013 SCC 67, [2013] 3 S.C.R. 895, at paras. 26‑27; see alsoDunsmuir, at para. 55).

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Although my colleague Côté J. does not call the reasonableness of the arbitrator’s decision into question, she finds that the standard of correctness should apply to it instead. On this point, her concurring reasons stray, in my humble opinion, from the Court’s decisions in Nor‑Man, Alberta Teachers and Dunsmuir, among others. The questions of evidence and procedure that arise here with respect to the principle that motives are “unknowable” and to deliberative secrecy in the context of an employer’s collective decision‑making authority are not outside the arbitrator’s area of expertise. Nor does the application of that principle and of deliberative secrecy to a fact situation characteristic of a dismissal amount to a question that is detrimental to consistency in the country’s fundamental legal order. Once this is established, maintaining that the concepts at issue do not fall solely within the arbitrator’s expertise in the area or jurisdiction over the matter (paras. 82 and 84 of my colleague’s reasons), or that one of them is a general principle of law that applies to other legal fields (para. 82 of her reasons), is not in my opinion enough to justify dispensing with the deferential standard that is required in such a case: Nor‑Man, at para. 55, citing the majority in Smith, at para. 26, and Dunsmuir, at para. 60; Mowat, at para. 23.

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In concluding, I must make one final comment. In my humble opinion, it is most unfortunate that, more than six years after filing a grievance with respect to a dismissal, the Union has not yet been able to begin presenting its evidence. The mission of the grievance arbitration system, that is, to provide employers and employees with justice that is accessible, expeditious and effective, has been forgotten. I would note the importance of the sensible rule that, with only a few exceptions, a grievance arbitrator’s interlocutory decision, in particular one concerning evidence and procedure, is not subject to judicial review:Syndicat des salariés de Béton St‑Hubert — CSN v. Béton St‑Hubert inc., 2010 QCCA 2270, at para. 23 (CanLII); Sûreté du Québec v. Lussier, [1994] R.D.J. 470 (C.A.); Collège d’enseignement général et professionnel de Valleyfield v. Gauthier Cashman, [1984] R.D.J. 385 (C.A.). The courts of several provinces have taken a similar deferential approach to interlocutory decisions of arbitrators: Lethbridge Regional Police Service v. Lethbridge Police Association, 2013 ABCA 47, 542 A.R. 252, at para. 21; Canadian Nuclear Laboratories v. Int’l Union of Operating Engineers, Local 772, 2015 ONSC 3436, at paras. 5‑7 and 11 (CanLII); Blass v. University of Regina Faculty Assn., 2007 SKQB 470, 76 Admin. L.R. (4th) 262, at para. 82. In the instant case, the arbitrator had offered to hear the testimony of the executive committee’s members in camera (para. 22).That would in all probability have obviated any risk of consequences that would be impossible to correct at the time of the final award. The lengthy judicial review proceedings at the stage of an interlocutory decision that are now drawing to a close could then have been avoided.

I would therefore dismiss the appeal with costs throughout and remand the case to the arbitrator in order that the inquiry into the grievance may at long last proceed."

Note: The summary and body are drawn from Eugene Meehan’s SupremeAdvocacy Weekly Updates for the Law Community.