Yesterday I vented my spleen about a case where a judge had reduced the sentence of two crooked cops who admitted perverting the course of justice in a manslaughter case and failed to give reasons for it. A friend has since emailed me part of the Court of Appeal ruling in Lewis v Wilson & Horton  3 NZLR 546 (the billionaire name suppression case), which talks a little about this issue. Surprisingly, there is no general rule that judges must give reasons for their decisions under New Zealand law. However, the Court of Appeal - who are now the Supreme Court - thought it was a good idea for three reasons:
Most importantly, the provision of reasons by a Judge is an important part of openness in the administration of justice. The principle of open justice in criminal proceedings is affirmed by s138(1) of the Criminal Justice Act 1985 and s25(a) of the New Zealand Bill of Rights Act 1990, but it is far older in observance and extends beyond criminal proceedings (although it is of particular importance there). It yields only where the application of the general rule in the particular circumstances of the case would frustrate the interests of justice, and then only to the extent necessary...It was not necessary for the Court to rule on the issue in order to decide the case, but they expressed a desire to "at an early opportunity". Hopefully they'll get that opportunity sooner rather than later.
The principle of open justice serves a wider purpose than the interests represented in the particular case. It is critical to the maintenance of public confidence in the system of justice. Without reasons, it may not be possible to understand why judicial authority has been used in a particular way. The public is excluded from decision-making in the courts. Judicial accountability, which is maintained primarily through the requirement that justice be administered in public, is undermined.
The second main reason why it said Judges must give reasons is that failure to do so means that the lawfulness of what is done cannot be assessed by a court exercising supervisory jurisdiction. Those who exercise power must keep within the limits imposed by law. They must address the right questions and they must correctly apply the law. The assurance that they will do so is provided by the supervisory and appellate courts. It is fundamental to the rule of law. The supervisory jurisdiction is the means by which those affected by judicial orders, but who are not parties to the determination and who have no rights of appeal or rehearing, obtain redress. Their right to seek such review is affirmed by s27 of the New Zealand Bill of Rights 1990. It is important that sufficient reasons are given to enable someone affected to know why the decision was made and to be able to be satisfied that it was lawful. Without such obligation, the right to seek judicial review of a determination will in many cases be undermined.
The third main basis for giving reasons is that they provide a discipline for the Judge which is the best protection against wrong or arbitrary decisions and inconsistent delivery of justice. In the present case it is hard to believe that the Judge would have granted the order if he had formally marshalled his reasons for doing so.