Breach of the prosecutor's duties made the trial unfair in Stewart v R  NZSC 53 (28 May 2009). That meant that the proviso could not be applied. But even if the trial had been fair the convictions would have been quashed because the trial outcome may have been different without the errors.
There is no new law in this decision, just a reminder of the duties of prosecutors and illustrations of their breach. Cases applied are R v Roulston  2 NZLR 644 (CA), R v Hodges CA535/02, 19 August 2003, R v Cook  1 SCR 1113, R v Mallory (2007) 217 CCC(3d) 266 (Ontario CA), R v E CA308/06, 11 September 2007, and Lord Bingham's well known dictum in Randall v R  1 WLR 2237 at para 28 (noted here 30 June 2008). There is also a quotation from Matenga v R  NZCS 18 (13 March 2009) – a case that has been withheld from publication until completion of a retrial: does this breach the Court's own suppression order? (Of course not, don't be silly.)
Here the prosecutor had created substantial prejudice to the accused's right to a fair hearing by his criticism, in closing, of a defence expert witness. This was a senior psychiatrist, who the prosecutor, without evidential foundation, said had accepted money in an attempt to establish a defence, had spoken "psychobabble", and had "[bent] things around to suit the accused". This was an appeal to prejudice through the use of emotive and inflammatory language, which was made all the worse because the witness was in general agreement with the Crown's expert. The Judge may not have been able to remedy this prejudice, but in any event he had increased it by referring to the witness's evidence as crucial and mentioned without disapproval the Crown's submission that the witness's conclusion was "patently ridiculous".
Another important breach by the prosecutor was alleging, without an evidential foundation, that the accused and his witnesses were lying in order to avoid his being convicted. This required an assumption of guilt as the motive for lying, and so subverted the presumption of innocence.
This point was not affected by the accused having alleged that prosecution witnesses had their own reasons for lying, because no presumption of innocence applies to them.
The Crown did not seek a retrial, as the appellant had served his sentence. This does not always prevent a retrial, as is illustrated by a high-profile (well, world famous in New Zealand) retrial currently under way. I say no more ... hush ma mouf.