Most criminal convictions in New Zealand are safe, but the problem is that there are always going to be one or two that are not, Supreme Court Justice William Young has said.

Speaking to the Newshub programme The Nation on calls for a criminal cases review commission, Justice Young said most of the "clean exoneration" cases that he was aware of were actually picked up by the Court of Appeal the first time round.

"But not all of them. And Teina Pora is an example," he said.

Asked if there was a judicial reluctance to say that sometimes the system had got it wrong, Justice Young said this was a hard allegation to refute.

"Judges are normally pretty quick to recognise that others have got it wrong, and I haven't myself experienced a judge in a case where there was a criminal conviction in issue saying 'well, crikey we can't allow this appeal because the criminal justice system will be put into disrepute'. On the whole we allow lots of appeals and I think that most judges will accept that an appeal system and a recognition of error and a correction of error is part of a criminal justice system."

There are no powers to compulsorily compel evidence, and once the appeal process is over, the power to subpoena witnesses is gone, he said. As he understood it, the Ministry of Justice had no powers of compulsion and its investigations were pretty much confined to asking the Police to reinvestigate on occasion.

"It's a fact that post conviction appeal there isn't a state facilitated and funding system for fathering evidence. The music has stopped at that point. Now maybe it stops too soon; on the other hand, it's got to stop sooner or later. There will always come a point where the state is going to stop funding and facilitating claims of innocence."

When asked for his views on establishment of a criminal case review commission, Justice Young said his view - and it was one that the Chief Justice shared - was that they did not oppose it.

"The current process is based on assessments carried out by the Ministry of Justice. There's been quite a lot of work done about that; my exposure is limited to the cases that I've heard which have been referred to the Court of Appeal by it. They don't investigate cases, in the sense that a criminal case review commission might; rather they respond in a reactive way to what the defendant or person seeking a review of conviction puts up.

"So if that's a well-resourced defendant who has a private investigator and access to lawyers and forensic scientists if that's appropriate ... the case will be put up properly and will be addressed. If they haven't, there isn't legal aid and there isn't state funding; that's an argument that is certainly available in favour of the setting up of some sort of process or changing the process."

Justice Young said he would support the establishment of a review commission in principle.

"I'm not really in a position to make an assessment of a cost-benefit analysis, which would be for others, but I would be very happy if there was a ... criminal case review commission set up."