British backpacker Grace Millane’s murderer was last week sentenced to life imprisonment, with a minimum non-parole period of 17 years.

However, the killer’s identity has not been published in New Zealand. When convicting Millane’s killer, Justice Simon Moore extended an existing suppression order that not only prohibits his naming, but also the reasons for making it.

Why is this so? Surely if someone is being tried for a crime and the public can freely sit in the court to watch events unfold, then it should be fully reported without restriction? Especially, you might think, after the trial has ended and a verdict has been pronounced.

New Zealand has a history of suppressing the publication of information about those facing trial, and even after conviction, that is quite different to many other jurisdictions. Although open justice always has been important, New Zealand is a small country where everyone seems to know everyone else and so that puts a premium on controlling information.

Publish that someone is facing trial or has been convicted of an offence, and the entire country may quickly learn of it. Anything else that person might have done in the past could then filter into the jury room. And there’s no heaving crowds in which to hide from the social stigma of a criminal charge or subsequent conviction.

The New Zealand Law Commission considered such historical reasons for controlling trial information in 2008. Following from its report, the Criminal Procedure Act 2011 then reformed when suppression orders may be given.

It empowers a judge to issue an order forbidding the publication of identifying details for people charged with, convicted or acquitted of an offence.

Such suppression orders are mandatory in relation to some sexual offences, if the publication of an accused’s identity would lead to a victim’s identification. For any other offending, orders only can be issued if publication would have at least one of a set of specific consequences.

These include that publication would be likely to cause extreme hardship to the defendant or a person connected with the defendant, cast suspicion on another person, cause undue hardship for a victim, create a real risk of an unfair trial or endanger someone’s safety.

Even where such consequences are established, the court can still decline to make a suppression order if countervailing interests prevail. And so most often name suppression only lasts the duration of the trial and is lifted upon a conviction.

That’s because the court of appeal has on a number of occasions emphasised that “the test [for permanent suppression] … is a demanding one. It places the emphasis very much on the importance of an open justice system and requires a real risk of ‘extreme hardship’ to be established before the court may consider suppression”.

In particular, addressing public concern that name suppression was too easily available for the rich and famous, the Criminal Justice Act specifically states that: “The fact that a defendant is well known does not, of itself, mean that publication of his or her name will result in extreme hardship.”

Consequently, the number of people winning permanent name suppression in New Zealand’s courts has declined by some 40% in the last decade. Far from being an increasingly common phenomenon, permanent name suppression is actually a practice on the slide.

Whether it is a practice that can survive in an era of Facebook, Google and Instagram is another question altogether. While the justice minister Andrew Little is endeavouring to create an agreement that enforces suppression orders on overseas media, the behemoths of the internet are harder to tame.

In particular, their ability to operate in New Zealand from behind the protection of the US first amendment means there are very few legal sanctions this country can bring to bear on them. Meaning that any compliance with our courts’ suppression orders is, in effect, voluntary.

And if the state’s effective control over information depends upon internet companies and websites being prepared to do the right thing, well, that’s really not a very firm basis at all.

• Andrew Geddis is a professor of law at Otago University