What is the burden of persuasion for a party seeking to tender an undisclosed document?

The party seeking to tender the undisclosed document bears the burden of persuasion, see Carol v. Gabriel (1997), 14 C.P.C. (4th) 376 (S.C.) per Henderson J.

Part of this burden is to establish, to the court's satisfaction, a reasonable explanation for the failure to disclose. As Henderson J. stated in Carol v. Gabriel, supra: ...Even in cases where no prejudice will ensue from the admission in evidence of the document, it will be excluded unless there is a reasonable justification for the earlier failure to disclose it. To hold otherwise would be to dilute the disclosure obligation and tempt counsel to refrain from disclosing in situations where they do not expect any prejudice to result.