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Edelman J

purpose. The requirement of proportionality is a requirement that the law imposing detention does so in a manner that is reasonably capable of being seen as necessary for a legitimate purpose. This requirement was established in the decision of Lim.

103 In Lim, the joint judgment of Brennan, Deane and Dawson JJ (with whom Gaudron J relevantly agreed ) recognised that it is a legitimate purpose of the exercise of power under s 51(xix) of the Constitution for the Commonwealth Parliament to confer upon the executive "authority to detain (or to direct the detention of) an alien in custody for the purposes of expulsion or deportation". But, their Honours explained, laws that provide for the purpose of deportation will only be valid "if the detention which they require and authorize is limited to what is reasonably capable of being seen as necessary for the purpose[] of deportation". If the detention is not reasonably capable of being seen as necessary for the purpose of deportation, it was said, the law "will be of a punitive nature and contravene Ch III's insistence that the judicial power of the Commonwealth be vested exclusively in the courts which it designates".

104 This proportionality limit on Parliament's ability to create executive power to detain reflects the jealous treatment of individual liberty as generally the province of the judiciary and, at a high level of generality, informs the constitutional separation of powers. The concern is generally "whether there are alternative, reasonably practicable, means of achieving the same object but which