The recent wrongful detention of two Australian citizens by immigration authorities highlights that our Constitution offers inadequate protection for freedom of the person.

This is not the first time Australian citizens have been unlawfully detained. In 2001, Vivian Solon, who had suffered a head injury, was deported even though she told immigration officials she was an Australian citizen. In 2004, Cornelia Rau, also an Australian citizen, was held in immigration detention after she was unable to identify herself because of mental illness.

The government could do this because migration legislation does not require judicial authorisation for a person to be deprived of their freedom. The Solon and Rau cases were found to be only two of more than 247 instances of unlawful detention that had occurred over the previous 14 years. The extent of the government's power was revealed in 2015, when the Department of Immigration announced Operation Fortitude. This would have involved stopping people randomly on Melbourne's streets to check their migration status. The operation was cancelled only after mass public protest.

So, anyone who is walking in the Melbourne CBD and speaks with a strange accent, or has suffered a brain injury, or is experiencing mental illness and cannot demonstrate a right to be in Australia, is liable to detention at best – and deportation at worst – without recourse to the courts.

Where the Constitution lacks

The reason the government has this power is because Australia's Constitution does not adequately protect individual liberty.

In 1992, the High Court held that separation of powers means that only courts can declare people guilty of crimes and imprison them. It therefore held that parliament cannot enact laws authorising the government to do that.

However, the court said that parliament can authorise the government to order so-called "non-punitive" detention - for example, detention for immigration purposes or in cases of communicable diseases.

Section 75(v) of the Constitution allows someone to challenge government decisions on administrative grounds. However, the High Court has held that this section does not allow the courts to decide whether the exercise of power is reasonable. On this basis, it found it would be lawful to detain someone under the Migration Act forever.

The court has also held that there may be many other – undefined – circumstances in which people can be detained without court approval.

The concept of "non-punitive" detention is vague. It is also oxymoronic: all detention is surely punitive to the person who experiences it. It leads to the bizarre situation that the law provides you more protection if you have committed a crime than if you have not.

It is fundamental in a free society that the law should not allow the state to deprive a person of liberty other than through due judicial process.

What a protection could look like

The Liberal Party proclaims its belief in "the inalienable rights and freedoms of all peoples" and a "just and humane society". Yet it marked the 800th anniversary of the Magna Carta – the founding document protecting rights in western democracies – by drafting legislation authorising deprivation of citizenship without the need to go to court.

Labor has been no less resistant to the idea of constitutional rights. The Rudd government's terms of reference to its inquiry into human rights specifically excluded consideration of putting new rights into the Constitution.

Opposition by politicians to constitutional rights is obviously self-serving, and it is often absurd as well. Former NSW premier Bob Carr objected to a Bill of Rights on the ground that it would create a "lawyer's picnic".

In a free society, it ought never to be lawful for a government to detain people by executive order alone.

The only effective way to protect liberty of the person is to deny the government the power to detain, unless it can demonstrate to a court that there are reasonable grounds for deprivation of liberty. And the only effective way to prevent the government from enacting legislation to give itself that power is to create a constitutional right protecting freedom of the person.

This right could be phrased as:

Everyone has the right to due process of law and not to be unreasonably deprived of personal liberty.

In a system where the burden appears to lie on the individual to prove they are lawfully in Australia rather than on the state to prove they are not, we are all vulnerable to deprivation of liberty.

The right to individual liberty is also a basic requirement of human dignity. That a person has been deprived of a right by a democratically elected parliament does not diminish the assault on their dignity.

The concept of a free society inescapably requires that limits be imposed on the will of the majority. That is why the power of parliaments has to be restrained.

This is what opponents of rights who trot out the objection to "unelected judges" overturning parliament's will fail to grasp. It is precisely because judges are independent of the will of political majorities that ultimately only the courts can effectively protect individual freedom.