The New Zealand Court of Appeal is this week hearing a case that is unusual in a number of respects.

The person bringing it is Brenton Harrison Tarrant, the 35-year-old Australian man convicted and sentenced for the murder of 51 people in Christchurch in March 2019.

Tarrant – who earlier pleaded guilty to those murders, along with attempting to murder 40 others and committing the acts as terrorism – is seeking to reopen his conviction and sentence.

At first glance, this might seem baffling: how can a person who earlier admitted to serious crimes – and who was sentenced years ago – be trying to appeal? The answer lies in some important rules that illustrate how New Zealand’s legal system works.

Legal systems have to balance competing principles. One principle is that decisions by criminal courts should be final, so that those affected can move on.

At the same time, it is important the decision is the right one, since an incorrect finding by a criminal court is problematic.

As a result, those convicted have a right to appeal, but the Criminal Procedure Act 2011 gives them 20 days to file their application. Additionally, the Appeal Court can extend that time limit if there are good reasons to override the need for finality.

So, the hearing now before the court is actually an application for an extension of time. Tarrant’s notice........

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