The High Court has issued one of its most important decisions in recent memory. In the case of NZYQ, the court overturned its 2004 judgment in Al-Kateb, which held that the parliament could authorise the indefinite detention of non-citizens, as long as there was some prospect that the person could be removed or deported, even in the long-distant future.

This time, the court ruled that even for serious offenders, immigration detention has to be reasonably necessary to achieve actual removal or deportation. This is not about the time the offenders serve in prison as punishment for the offences they committed. That is a separate process, not part of the immigration calculus.

You know democracy is in trouble when governments start ignoring court decisions, or the courts are called “enemies of the people” for their role in upholding the Constitution.

Hence, if a person is stateless or a refugee who cannot be returned to their country of origin, and no country will take them, they cannot be detained simply based on the hope that, one day, they might be removed from Australia. Instead, alternatives, such as a system of monitoring in the community, or possibly time-limited, preventative detention, have to be considered.

The government is doing just that: passing and considering further legislation that would uphold constitutional requirements, while also reducing risks from the unconditional release of unlawful non-citizens convicted of serious criminal offences. The opposition, however, is not satisfied.

Opposition Leader Peter Dutton and his immigration spokesman, Dan Tehan, say the government should be putting all these offenders back into detention, and that the government should have been preparing to do so for the past few months.

But almost no one expected the High Court’s decision to come down this way, at least when it did. That includes legal scholars and experts working in the field. The decision was unanimous, but it also followed years of failed attempts to persuade the court to rule as it did in this case.

The court makes clear that the indefinite, unconditional detention of unlawful non-citizens is unconstitutional. Ongoing detention is only permissible if it is part of an active effort to persuade another country to accept a detainee, in the near term. And so the best that could be achieved, from a public safety perspective, is a regime that allows short-term, renewable periods of detention for the worst offenders, while we try to persuade friends and allies overseas to take them off our hands.

The more realistic option is to adopt the same model that applies to all serious offenders: make them serve their time in prison, for the offence for which they have been convicted, and then monitor them closely when they are released. In an immigration context, that has the added virtue of making sure that unlawful non-citizens can be found, should they later become deportable.