What constitutes denial of natural justice?

However, we do not agree with the submission of the Commission that this error amounted to a denial of natural justice of the kind discussed in Université du Québec à Trois-Rivières v. Larocque, 1993 CanLII 162 (SCC), [1993] 1 S.C.R. 471 at p. 491, where Lamer C.J. stated: 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.