He has proposed various "safeguards" but the proposal remains fundamentally flawed and, depending on how it is framed, unconstitutional.

Yes, we must respond to the new horror of random attacks on innocent people, but we should do so within the principles of the rule of law and the separation of powers – the bedrock of our democracy.

Turnbull's new proposals seek to deal only with people who have been convicted. And therein lies a better solution.

That solution lies in penalty and sentencing. We should legislate to attach life sentences to terrorism offences. It is, after all, attempted murder. The legislation could then set suggested non-parole periods.

This would sit much better with standard sentencing practice. Sentencing judges usually take into account things like the need to punish, the possibility of reoffending, remorse (if any), the prospects of rehabilitation (if any), good character, previous offences and so on.

In the case of religious, fanatic terrorism, there is no remorse and there is a high prospect of reoffending and problematic prospects of rehabilitation. That justifies longer sentences.

Higher sentences are also justified because of the randomness of the act. If, for example, someone kills or attempts to kill with a motive directed at a particular victim – say in revenge, anger, jealousy, or to remove a drug-dealing competitor and so on – then the deed is done, so there is less likelihood of reoffending than in the case of terrorist crime.

In case of the latter the motive is to, say, destroy or completely transform Australian society. The motive remains indefinite so the prospect of reoffending is very high.

That said, everyone should have a chance of redemption – hence the non-parole period. When that expires, the onus would be on the offender to prove they have rehabilitated.

Importantly, these people will have been convicted after a fair trial, having been properly represented and having the evidence against them properly tested.

The trouble with the Turnbull proposal (although we have not seen the details yet) is that is smacks of similar laws to deal with two similarly fear-inducing offenders – paedophiles and bikies.

In those cases, state legislatures attempted to provide for detention after sentences were fully served on "public safety" grounds. The various pieces of legislation were challenged in the High Court – usually successfully.

In deciding these cases, the High Court laid out some constitutional principles which will make Turnbull's task very difficult. It could develop the principles even further if the terrorism legislation is challenged.

Australia, unlike the US and most other liberal democracies, does not have a Bill of Rights with a "due process" clause that would make incarceration without trial (other than in cases of mental illness or communicable disease) unthinkable.

However, our Constitution does provide some protections. First, it vests the "judicial power of the Commonwealth" exclusively in the High Court, other federal courts created by the Commonwealth Parliament, and any state court that the Federal Parliament vests with Federal jurisdiction – for example, the power to hear tax and copyright cases.

The High Court has held that these courts cannot exercise any power which is not "judicial" in nature. They cannot legislate (for everyone) but only hear judicially cases restricted to the parties before them.

Federal courts (and state courts that have any power to exercise any Federal jurisdiction) are restricted to resolving disputes between parties – bringing matters to finality. The parties can be individuals, corporations or governments.

These courts have to act "judicially". It means they have to give parties a hearing and the right to bring evidence and test evidence brought against them.

Most of the bikie and paedophile laws on incarceration after sentence have fallen foul of these principles.

These principles could be developed further. A couple of academic papers have, in reference to civil cases, pointed to the nature of the judicial power as one that brings disputes to finality. And it is important for society that disputes are finally resolved.

But the principle should also apply in criminal cases. After the sentence is served the government should not be able to have another crack at it based on the same crime.

This is why we have the principle of no double jeopardy.

Sure, there may be an argument (but not much of one) for reopening a case of acquittal if new facts are found. However, reopening a case after conviction and the sentence being served is another level of egregiousness.

The government could possibly argue that it is not in fact reopening a case after its final disposal, but rather starting a new case altogether – one that says: you are a menace so we are locking you up.

If that is the case it should apply universally – not just to people who have been convicted. And that would be a very dangerous proposition. It would mean the executive government could order the incarceration of people without trial.

The risk of abuse is obvious. As is the slippery slope. Further, such a thing would be unconstitutional because locking people up would be treading into the sphere of the judicial power which can only be exercised by judges – judges who are independent, appointed till age 70 and only removable upon proven misbehaviour and a vote of Parliament.

Such a power should not be exercised by politicians beholden to an electorate.

There is another difficulty here. Once you imprison people without judicial supervision (over a trial or a remand in custody pending trial) they are no longer "convicts" or "remandees", but they are still "prisoners".

They become like prisoners of war and gain a form of legitimacy, at least in the eyes of their followers. That is what happened in Northern Ireland.

They will be seen as martyrs.

I do not want to underestimate the danger these people present or the evil they have perpetrated. But if they are still a danger at the end of their sentences then the sentence was not long enough or the post-sentencing surveillance by law-enforcement is not good enough.

Therein lies the answer – not in the undermining of our criminal justice system and our liberal democratic society.

www.crispinhull.com.au