In 2004 The High Court handed down a number of decisions concerning detention imposed for purposes allegedly unrelated to punishment. This paper outlines the way the Federal Constitution restricts (and also facilitates) the imposition of "non punitive detention" by our governments. Such laws (as passed by the Federal Legislature) are constitutionally valid provided they can be characterised as falling within a legislative head of power under section 51 off he Constitution. The power to detain for non punitive purposes can be reposed by the Legislature in the either the Executive or Judicial arms of government. Detention by the Executive is non punitive (and therefore does not offend the separation of powers) even though it involves a deprivation of liberty, provided it is imposed for “legitimate non punitive purposes”. Legitimacy is in turn determined by reference to the section 51 heads of power. Detention for non punitive purposes by the judicial arm of government is constitutionally valid provided that (i) a “judicial process ” is adopted and (ii) (arguably) there is some link (albeit tenuous) with a previous finding of criminal guilt. The continuing existence of the “constitutional immunity ”from being detained by other than judicial order identified by the High Court in its 1992 decision in Lim v Minister for Immigration is called into question.