R. v. St. Cloud,  2 S.C.R. 328 is the leading case from the Supreme Court of Canada with respect to the application and interpretation of the bail review provisions in s. 520(1) of the Criminal Code.
Justice Wagner, as he was then, speaking for a unanimous Court, provides the following helpful guidance beginning at para. 120:
On the basis of the wording of ss. 520 and 521 Cr. C. …I conclude that these sections do not confer on the reviewing judge an open-ended power to review the initial order respecting the detention or release of the accused. The reviewing judge must therefore determine whether it is appropriate to exercise this power of review.
It will be appropriate to intervene if the justice has erred in law. It will also be appropriate for the reviewing judge to exercise this power if the impugned decision was clearly inappropriate, that is, if the justice who rendered it gave excessive weight to one relevant factor or insufficient weight to another. The reviewing judge therefore does not have the power to interfere with the initial decision simply because he or she would have weighed the relevant factors differently. I reiterate that the relevant factors are not limited to the ones expressly specified in s. 515(10)(c) Cr. C. Finally, where new evidence is submitted by the accused or the prosecutor as permitted by ss. 520 and 521 Cr. C., the reviewing judge may vary the initial decision if that evidence shows a material and relevant change in the circumstances of the case.
With respect to the scope of what constitutes ‘new evidence’ at a bail review, Justice Wagner, beginning at para. 127, states:
I am …of the opinion that the reason why detained persons may not always tender all possible evidence at their first hearing lies in the generally expeditious nature of the release process and in the consequences of that nature, namely the short time between arrest and hearing, a lack of representation for accused persons, and incomplete evidence at this stage. The interests of justice would therefore be undermined if courts acting under ss. 520 and 521 Cr. C. were to adopt a narrow view regarding the "new evidence" that can be admitted under those sections.
In Palmer v. The Queen,  1 S.C.R. 759, at p. 775, this Court established the following criteria that must be met for evidence to be considered "new evidence" on appeal:
(1)The evidence should generally not be admitted if, by due diligence, it could have been adduced at trial ... .
(2)The evidence must be relevant in the sense that it bears upon a decisive or potentially decisive issue in the trial.
(3)The evidence must be credible in the sense that it is reasonably capable of belief, and
(4)It must be such that if believed it could reasonably, when taken with the other evidence adduced at trial, be expected to have affected the result. (citation omitted)
In my opinion, the four criteria from Palmer are relevant, with any necessary modifications, to the determination of what constitutes new evidence for the purposes of the review provided for in ss. 520 and 521 Cr. C. Given the generally expeditious nature of the interim release process and the risks of violating the rights of the accused, and since the release hearing takes place at the very start of criminal proceedings and not at the end like the sentence appeal, a reviewing judge must be flexible in applying these four criteria. I reiterate at the outset that the rules of evidence are relaxed in the context of the release hearing: s. 518 Cr. C.