Because New Zealand has an unwritten, largely informal constitution, it can change in quite major ways without generating much fanfare.

One such development took place last Friday with the delivery of the High Court's judgment in Taylor v Attorney-General. This case involved a challenge by five prisoners to a 2010 law that prevents all sentenced prisoners from voting. (Before 2010, only prisoners sentenced to three or more years in jail were stopped from doing so.)

After hearing this challenge, Justice Heath concluded that the 2010 law limits prisoners' right to vote as guaranteed under the New Zealand Bill of Rights Act, and does so in a way that cannot be "demonstrably justified in a free and democratic society." That finding was no real surprise; the Attorney-General already had warned Parliament about the problem before the law was passed.

After all, the effect of the law is to say that a person sentenced to prison cannot vote while someone sentenced to home detention for committing the exact same crime can do so. Or a person sentenced to five years in prison the week after an election is held will in practice be able to vote at the next one when released on parole, but a person sentenced to a month's imprisonment the week before that election will not. And so on.

Even if you think that some group of criminal offenders ought to lose their right to vote because of their manifest disrespect for societal norms, you still must admit that a law that takes away some people's voting rights on a more-or-less arbitrary basis is not a good idea. Which is why virtually everyone who has looked seriously at this law thinks it is fatally flawed.

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The novelty lies in what Justice Heath chose to do about these legislative problems. For after finding that the law barring prisoners from voting has numerous irrational and arbitrary consequences, he issued a formal declaration that it breaches the Bill of Rights Act.

Exactly what this means needs a bit of explanation. The declaration doesn't set aside the prisoner-voting law or permit prisoners vote. The Bill of Rights Act expressly prevents courts from overturning parliamentary enactments in this way.

But it does mark the court's formal and express finding that Parliament has passed a law that removes a fundamental right - perhaps the most fundamental civil right - from literally thousands of New Zealanders. And it has done so in a way that lacks any measure of rational justification.

Such a declaration really matters a lot. First of all, it marks a real departure from the approach that New Zealand courts traditionally take towards Parliament.

Judges will usually say that their job is to just apply the law as Parliament intends it to be. They may on occasion use comments in their judgments to suggest to Parliament that it might wish to reconsider some matter, but in general the courts don't get into the business of assessing the wisdom of parliamentary enactments.

Justice Heath's decision to issue a declaration reverses that position. His honour is unequivocally and explicitly telling Parliament that its 2010 law is a bad one because it fails to meet the standards Parliament itself established when passing the Bill of Rights Act.

Second, this message matters for all of us. His honour is alerting us to the fact that by not meeting these standards, Parliament has failed to do its most important job.

As citizens, we elect members of Parliament to make laws on our behalf. When they do so, we'd like them to put in place the kind of policies that we think are best. That's why we vote for one political party over another.

But we also want all MPs to act as good and trustworthy custodians of the lawmaking powers they've been granted. A part of that is making sure they respect the rights that we all possess, making sure that these are limited only in ways that are reasonable and have been carefully considered.

So when a High Court judge comes out and makes a formal declaration that Parliament has failed to do this, then we really should sit up and pay attention.

It doesn't matter that the rights in question are those of prisoners. That's the thing about rights: either they matter for everyone, or they don't matter at all.

Justice Heath's judgment now poses a challenge to Parliament. Its Justice and Electoral Committee currently is holding an inquiry into the 2014 election. This inquiry inevitably will result in a bill to make some changes to the Electoral Act for the 2017 election.

Will the Government heed the unequivocal message that Heath has delivered and use this bill to reopen the issue of which prisoners ought to be permitted to vote? And do we, as citizens, care enough about our democracy to demand that it do so?

Professor Andrew Geddis teaches at the Faculty of Law at the University of Otago.