The opposition procedure in Australian patent law is an effective tool for improving the quality of granted patents. The current, pre-grant, process is, however, open to abuse by opponents who merely wish to delay the grant of a patent. Received wisdom has it that a post-grant procedure would be contrary to the Australian Constitution - that is, for a delegate of the Comissioner of Patents to decide to an opposition post-grant would be an improper exercise of judicial power. The paper details the various tests for judicial power to assess the truth of this wisdom. The conslusion, after a review of the High Court precedents and commentary, is that a post-grant opposition procedure, assuming it is substantially similar to the current pre-grant process, would not offend the Constitution.