When is an office or public trust 'under the Commonwealth' for the purposes of the religious tests clause of the Australian Constitution?

The religious tests clause of s 116 of the Australian Constitution prohibits religious tests for any office or public trust 'under the Commonwealth'.1 The few cases decided by the High Court concerning the religious tests clause, including most recently Williams v Commonwealth ('School Chaplains Case'),2 provide no explanation of what the expression 'under the Commonwealth' might mean. This paper seeks to develop an interpretation of the expression 'under the Commonwealth' as it is used in the religious tests clause that is meaningful, avoids undesirable and perverse outcomes, reconciles the existing cases and is consistent with s 116's drafting history. The paper argues that an office or public trust will be 'under the Commonwealth' for the purposes of the religious tests clause when the office or public trust stands in a familial relationship with the federal government, understood as encompassing not just its executive arm but also its legislative and judicial arms.