In Pochi v Macphee  HCA 60 the argument proceeded on the assumption that any person who is a British subject under the law of the United Kingdom cannot be an alien within section 51(xix) of the Constitution, which the court held was incorrect.
“The allegiance which Australians owe to Her Majesty is owed not as British subjects but as subjects of the Queen of Australia. Now, once the British Nationality Act 1981 (U.K.) has come into force, the principle that every Commonwealth citizen is a British subject will have finally been abandoned, and the status of British subject will be restricted to a narrow group. If English law governed the question who are aliens within s. 51(xix), almost all Australian citizens, born in Australia, would in future be aliens within that provision. The absurdity of such a result would be manifest. The meaning of “aliens” in the Constitution cannot depend on the law of England. It must depend on the law of Australia.”