Is a failure to admit or deny relevant evidence a breach of natural justice?

In paragraphs 44 and 46 of the Université du Québec à Trois‑Rivières v. Larocque judgment, Lamer C.J. wrote: 44 The proposition that any refusal to admit relevant evidence is in the context of a grievance arbitration a breach of natural justice is one which could have serious consequences. It in effect means that the arbitrator does not have the power to decide in a final and exclusive way what evidence will be relevant to the issue presented to him. That may seem incompatible with the very wide measure of autonomy which the legislature intended to give grievance arbitrators in settling disputes within their jurisdiction and the attitude of restraint demonstrated by the courts toward the decisions of administrative bodies. . . 46 For my part, I am not prepared to say that the rejection of relevant evidence is automatically a breach of natural justice. A grievance arbitrator is in a privileged position to assess the relevance of evidence presented to him and I do not think it is desirable for the courts, in the guise of protecting the right of parties to be heard, to substitute their own assessment of the evidence for that of the grievance arbitrator. It may happen, however, that the rejection of relevant evidence has such an impact on the fairness of the proceeding, leading unavoidably to the conclusion that there has been a breach of natural justice.