What is the test for adding affidavit evidence to the record of a decision-maker in a judicial review?

Under CPR 7, the practice is continued of judicial reviews being conducted on the basis of the record of the decision-maker whose decision is under review. CPR 7.09 requires that the decision-maker provide a record of the process, which may include reasons for the decision. Under CPR 7, the evidence on a judicial review is limited to the record, except for certain limited purposes. CPR 7.28 requires that a party who proposes to introduce evidence beyond the record must bring a motion for its admission. A grant of permission to include affidavit evidence to supplement the record is rare and its scope limited. Per the Court in Canadian National Railway v. Teamsters Canada Rail, 2017 NSSC 10:

It is well established that admitting evidence beyond the record in a judicial review proceeding will only be permitted in exceptional circumstances. The general rule is that affidavit evidence which was not before the decision-maker below should not be used to supplement the record on review. There are a few exceptions to that general rule. This Court in Sipekne'katik v. Nova Scotia (Minister of Environment), 2016 NSSC 260 (Sipekne'katik) recently confirmed that these exceptions apply only in “exceptional circumstances.”