In R v Gwaze  NZSC 52 (17 May 2010) a retrial was ordered because some defence evidence had been admitted wrongly and there had been no proper opportunity at trial for the Crown to respond to it.
There is nothing particularly odd about that. Unusual, yes, because this was an appeal against an acquittal, and this seems to be the first time in New Zealand that a retrial has been ordered on this sort of appeal. Occasionally there are appeals on questions of law reserved during trials (s 380 Crimes Act 1961) – but success on these is not inevitably followed by retrial.
A number of interesting points were considered in this judgment of the Court delivered by the Chief Justice:
The rules concerning double jeopardy (s 26(2) New Zealand Bill of Rights Act 1990) do not apply in this context because there is a high hurdle (s 382(2)(b) Crimes Act 1961) for the prosecution to overcome before a retrial will be ordered and there is no need for the extra protection that the rules against double jeopardy afford (para 67).
A mistrial (s 382(2)(b) Crimes Act 1961) occurs where an error at trial is reasonably capable of affecting the verdict, in the sense that it is highly material to the verdict so that the integrity of the verdict was undermined by it (57, 61), applying R v Matenga  NZSC 18 (blogged here 9 July 2009 and 20 July 2009).
Question of law
A ruling as to the admissibility of evidence is a ruling on a question of law (52). This should be obvious, because admissibility is governed by statute (with minor exceptions) and the application of statute is a matter of law. To get to that conclusion, however, an unnecessary change in terminology was made: the decision to admit or exclude evidence is no longer to be called a discretion: it is an exercise of judgment (49, 51). The description in R v Leonard  2 NZLR 218, (2007) 23 CRNZ 624 (CA) para 22 of the balancing exercise in determining the admissibility of improperly obtained evidence as a "discretion" will have to be read in this new light.
In this case (and I do not discuss the facts as a retrial was ordered) the contested evidence should have been ruled inadmissible because it was unreliable hearsay (44), it was opinion that was not substantially helpful (46), and its prejudicial effect outweighed its (zero) probative value (48).
The Court held that the evidence was irrelevant (37-40). Strictly, there was no need for the Court to give other reasons for its being inadmissible, as s 7(2) Evidence Act 2006 excludes it. Even so, the evidence was irrelevant because it was not sufficiently connected to the facts in the trial. It was a preliminary assessment, recognising a possibility that a connection might emerge, but also recognising that there may be no connection, and that further inquiry was needed.