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Gageler CJ Gordon J Edelman J Steward J Gleeson J Jagot J Beech-Jones J

fluctuate, expression of the constitutional limitation in those terms would uncouple the limitation from its underlying constitutional justification. Demanding compliance with a limitation expressed in those terms would go beyond merely ensuring that the non-punitive purpose of detention remains a purpose capable of being achieved in fact. It would also go beyond merely ensuring that the detention is limited to what is reasonably capable of being seen as necessary for the purpose of removal.

Applying the constitutional limitation

If Al-Kateb was to be reopened and overruled, the defendants made a correct and important concession. The plaintiff having discharged an initial evidential burden of establishing that there was reason to suppose that his detention had ceased to be lawful by reason that it transgressed the applicable constitutional limitation on his detention, the defendants conceded that they bore the legal burden of proving that the constitutional limitation was not transgressed. The concession was correct having regard to the coincidence of two fundamental principles. The first, a principle of common law reflected in the traditional procedure for obtaining a writ of habeas corpus, is that where a person in the detention of another adduces sufficient evidence to put the lawfulness of that detention in issue, the legal burden of proof shifts to the other to establish the lawfulness of that detention. The second, a principle of constitutional law, is that "it is the duty of the Court in every constitutional case to be satisfied of every fact the existence of which is necessary in law to provide a constitutional basis for the legislation".

To establish that ss 189(1) and 196(1) of the Migration Act validly applied to authorise continuation of the plaintiff's detention, the defendants were accordingly required to prove that there existed a real prospect of his removal from Australia becoming practicable in the reasonably foreseeable future. Whilst the proof was required to be to a standard sufficient to support the making of a finding