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Edelman J

defendants couched the postulated purpose demonstrated its constitutional illegitimacy".

68 In effect, their Honours in NZYQ held that the purpose of the Commonwealth Parliament in ss 189(1) and 196(1) of the Migration Act was not, or was no longer, the legitimate purpose of removal of classes of aliens from Australia in the individual instances where it turned out that the purpose was not achievable as a matter of practicality. In those cases, as Professor Twomey correctly explained, the reasoning of the six members of this Court was that "if the legitimate purpose is effectively unachievable, then it is no longer the purpose, and the default [punitive] purpose of punishment springs back into place". Hence, six members of this Court relied on contravention of the first requirement to hold that ss 189(1) and 196(1) of the Migration Act should be disapplied.

69 On this point, I departed from the reasoning of the other members of the Court. I relied on the second requirement. There is a difference between a legitimate purpose and the manner in which that purpose is implemented. That is a difference between ends and means. The purpose of the Commonwealth Parliament in removing classes of aliens from Australia is legitimate, and consistent with the first requirement, even if in the implementation of that purpose removal is not practicable in the reasonably foreseeable future in some cases. Indeed, it is common that a legislative purpose will not be able to be achieved in every instance of its application. Nevertheless, as Lim requires, the choice by the Commonwealth Parliament of the means by which its purposes will be implemented must respect the constitutional separation of powers and the exclusivity of the judicial power to punish.

70 To reiterate: the constitutional separation of powers was held in Lim to require that the means of detention chosen by the Commonwealth Parliament for the legitimate purpose of removal of classes of aliens from Australia must not be "punitive". The term "punitive" was used in a novel sense to describe those