It is difficult to imagine that any citizen, or group of busybodies, could get standing to go to court seeking orders to override such legislation. Perversely, however, it is possible that the separating partner of some future same-sex marriage might be able, in a dispute about property or incidents of the relationship, to claim that the ACT law was invalid.

The Gillard government changed ACT constitutional arrangements to remove the power of the Commonwealth executive government to disapprove ACT legislation by ministerial fiat. That power, instead, now rests with a resolution passed by both houses of the federal Parliament.

Any backbencher could move such a motion. As with Kevin Andrews' bill to override Northern Territory euthanasia legislation, the government might prefer that approach to that of a formal resolution from the government. This way Tony Abbott would have extra room to manoeuvre about the principle of same-sex marriage in terms of Coalition policy. He might well expect that even on a free vote, the ACT legislation could be overridden without its being seen as deliberate action plotted by him.

There are two other methods by which ACT legislation could be attacked. First, the Commonwealth could propose legislation purporting to ''cover the field'' on marriage matters. Such legislation, if obviously intended to disallow gay marriages or even civil unions in the style of gay marriages, would have the incidental effect of overriding any state or territorial legislation in the field. The constitution explicitly gives the Commonwealth power to make laws - including, if it wants, exclusive laws overriding state and territorial laws - over marriage and divorce, and another section of the constitution provides that if there is any conflict with state or territorial law, the Commonwealth law prevails.

In fact, the Commonwealth has already passed such a law, enshrining John Howard's view that marriage can be only between a man and a woman. It explicitly provides for non-recognition of same-sex marriages contracted overseas.

But some argue that this law does not cover the field, and thus that it does not, in its present formulation, outlaw state or territory legislation in the area. This is because it is founded on a definition at a time when judges might be prepared to rule (as they have in Canada and the United States) that the constitutional definition is wider. The Commonwealth, in short, is said to be covering the field for heterosexual relationships, but still leaving open the field for gay marriages.

Australia has no constitutional base - such as privacy, or a right to equal access to the law under a bill of rights as in Canada or the US - but a number of Australian judges, and senior counsel, have suggested that courts will, at least in due course, recognise gay marriage.

The third option open to the Commonwealth is to have new attorney-general George Brandis directly approach the High Court for a declaration of invalidity of the territory law - perhaps also of other laws, such as Tasmania, going in that direction. States could also do the same. In this a government has powers not open to a citizen, unless given standing by the courts.

The issue could also be obliquely litigated were someone to return from New Zealand, where same-sex marriage has been legislated for, to go to the courts to seek to enforce some right said to be bestowed by that marriage but not otherwise. Probably this could only be ''recognition'' - since there are, these days, few practical discriminations based on differences between a de jure or de facto relationship.