Su v Commonwealth of Australia; BS v Commonwealth of Australia

In the case of Su v Commonwealth of Australia; BS v Commonwealth of Australia, the plaintiffs, two children who had been detained in immigration detention, challenged the lawfulness of their detention and subsequent arrest and imprisonment. The Federal Court of Australia was tasked with determining whether the plaintiffs' detention and arrest were lawful and whether their detention created an "umbrella of lawfulness" which rendered their subsequent arrest and imprisonment lawful.

The court had to decide several legal issues, including whether the plaintiffs, who were lawfully in immigration detention, enjoyed a right to residual liberty. It also had to determine the content of this residual liberty, whether the plaintiffs were arrested contrary to statute, and if this made their arrest unlawful. Additionally, the court considered whether the fact that the plaintiffs were treated as adults by the Federal Police created a divergence in the legal understanding of the situation between the migration officers and the Federal Police. Importantly, the plaintiffs conceded that their subsequent period of incarceration in an adult gaol was not unlawful.

The court held that the plaintiffs' detention was lawful, but their arrest and subsequent imprisonment were not. The court found that the plaintiffs did enjoy a right to residual liberty, and that this right was not satisfied by their detention. The arrest of the plaintiffs was found to be contrary to statute, rendering it unlawful. The court also found that the plaintiffs' detention did not create an "umbrella of lawfulness" which would render their arrest lawful, and that the disagreement between the migration officers and the Federal Police was a red herring. Finally, the court rejected the defendants' objection to the plaintiffs conducting an alternative case in the course of their reply, finding that the alternative case had been effectively abandoned in the plaintiffs' written submissions and that there was no prejudice to the defendants.

The court found in favour of the plaintiffs and ordered the defendants to pay the plaintiffs' costs. The court also found that the plaintiffs were entitled to compensation for the unlawful aspects of their arrest and imprisonment, but did not specify the amount of compensation in its judgment.