To what extent does the Evidence Act 2006 [NZ] exclude the common law discretion to rule inadmissible evidence that was obtained, not through improper acts of officials, but through unfairness arising independently of officials?
People who can access the New Zealand Universities Law Review can find an interesting discussion of this by Don Mathieson QC, "Fair Criminal Trial and the Exclusion of 'Unfair Evidence'" (2013) 25 NZULR 739 (October 2013). Dr Mathieson analyses a Court of Appeal decision which is currently subject to a suppression order, but which the Court has allowed to be discussed in professional publications.
So I am rather constrained in what I can say about the case here, and nor should I quote much of what Dr Mathieson says. In Adams on Criminal Law – Evidence at EA30.09(9) the central point is summarised in this way:
"The Court held that when a defendant argues that evidence such as a statement has been improperly obtained by the police, admissibility of the evidence must be determined in accordance with s 30. However, when the defendant argues that, although not improperly obtained, it would nonetheless be unfair to admit evidence against the defendant, admissibility will be governed by the exercise of the Court's common law discretion, which continues after the Evidence Act 2006."
Dr Mathieson argues, in effect, that the Court's holding, summarised in the last sentence of that passage, is wrong. There is no such common law discretion, and even if there was it would now have been replaced by the provisions of the Evidence Act, in particular ss 6, 7 and 8, pursuant to either s 11(2) or s 12.
Section 12 is central to the reasoning, so I set it out here:
If there is no provision in this Act or any other enactment regulating the admission of any particular evidence or the relevant provisions deal with that question only in part, decisions about the admission of that evidence—
(a) must be made having regard to the purpose and the principles set out in sections 6, 7, and 8; and
(b) to the extent that the common law is consistent with the promotion of that purpose and those principles and is relevant to the decisions to be taken, must be made having regard to the common law.
Dr Mathieson's argument is that this replaces whatever common law there might be on a relevant topic with law that interprets the Act in a way that, to the extent permitted, has regard to the common law. The new law is not common law – in the sense of law invented by judges in an area not covered by legislation – but rather is, collectively, decisions under s 12.
Then, applying s 7 and s 8 to the evidence that was obtained by what I might call the 'non-official unfairness' in the case at hand: yes, the evidence is relevant, and no, it is not excluded pursuant to the s 8 discretion. There is no resort to any posited common law discretion.
The Court of Appeal's reasoning was (here I risk quoting  of the suppressed judgment):
"It would be inconsistent with the common law and the purpose of the Evidence Act which is to promote fairness to parties, to construe s 30 as excluding the common law discretion. The continued existence of the common law discretion is consistent with the purpose of promoting fairness in s 6(c) to parties, and the Court must have regard to that purpose under s 11(2). The exclusion of evidence on unfairness grounds can be seen as dealt with only "in part" (in terms of s 12)by s 30, so that decisions on the admission of evidence can still involve a consideration of what is fair to the parties, that is, irrespective of the provisions of s 30. We conclude that the common law discretion survives the Evidence Act, although s 30 governs those cases to which the section applies."
You can see that this is quite a different interpretation of the effect of s 12 than that advocated by Dr Mathieson. The "decisions about the admission of that evidence" remain, in the Court's view, common law decisions.
I wonder whether this matters, particularly where the common law on the point had been undeveloped. You could say it matters if the Act is more restrictive than the common law. The Court, developing the common law, applied by analogy such of the considerations specified in s 30 as were relevant, and concluded that the evidence was admissible. Dr Mathieson on the other hand would confine the considerations to those applicable under s 8.
A difficulty is that, pursuant to s 12, it is the "purpose and principles set out" in s 8 to which the court must "[have] regard". Section 12 does not simply apply s 8 to the issue. Identifying the relevant purposes and principles of s 8 is not a simple matter, as none are "set out". A rule is not the same thing as a principle. There are abstract concepts named in the section: "probative value", "unfairly prejudicial effect", "the proceeding", "take into account", "the right", "offer an effective defence". But the only principle apparent in s 8 is the principle that evidence must not be ruled admissible if it would be unfair to do so. Arguably this isn't even a principle, it is a rule ("must not"). But let's pretend it is a principle. Is this principle more restrictive than the common law?
But don't let my ramblings deter you from reading Dr Mathieson's splendid article. It is unfortunate that it is not more widely available.