Is denial of natural justice reviewable as a matter of law?

The denial of natural justice is reviewable as a matter of law. However, not every rejection of evidence is a denial of natural justice. As noted by Lamer C.J.C. in Université du Québec à Trois-Rivières v. Larocque (1993), 1993 CanLII 162 (SCC), 101 D.L.R. (4th) 494 at 508 (S.C.C.) in the context of a grievance arbitration: 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.