Need witness competency be an issue separate from fairness?

The Supreme Court of Canada has split on the requirements for witness competency: R v D.A.I. 2012 SCC 5 (10 February 2012). The issue in Canada is one of statutory interpretation, and here revolved around whether an adult but mentally impaired witness needed to show an appreciation of the significance of a promise to tell the truth. The majority held that such appreciation did not need to be demonstrated because it could require some difficult abstract concepts. The policy of allowing impaired victims access to justice was of great importance.

Plainly, if the witness's response to questioning while giving evidence was such as to deprive the right of the defence to confront the witness, there would be trial fairness issues. However at the threshold stage the issue was whether the witness showed understanding of the oath or affirmation, and was able to communicate the evidence.

We in New Zealand do not have a competency requirement for witnesses. The idea is that "No person, whether on the grounds of age, intellectual disability, or mental disorder, or on any other ground, may be disbarred from giving evidence on the ground of incompetence. ... In the case of witnesses whose testimony is unhelpful – because of incoherence, for example – the judge may still exclude that evidence under the general exclusionary provisions in s 8": NZLC R55 "Evidence" – Vol 2 at C294.