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:
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 There is no such common law discretion, and even if there was it would now have been replaced by the provisions of the Evidence Act, in particular ss 6, 7 and 8, pursuant to either s 11(2) or s 12.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:
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):
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.