As soon as the ACT's law had been passed, the Commonwealth launched a challenge to it in the High Court, which delivered judgment on 12 December 2013. As to the relation between the ACT act and federal legislation, the Court found that the ACT act was invalid and of "no effect", because it was "inconsistent", in terms of the Australian Capital Territory Self-Government Act 1988 (Cth), and the federal Marriage Act 1961 (Cth). It was inconsistent both because its definition of marriage conflicted with that in the federal act and because the federal act was exclusive, leaving no room for any other definition in the legislation of a state or a territory. However, the Court went on to determine that the word "marriage" in Constitution s51(xxi) includes same-sex marriage, thus clarifying that there is no constitutional impediment to the Commonwealth legislating for same-sex marriage in the future. It can do so by amending the definition of "marriage" in the Marriage Act, which it did in December 2017.
