Between the nondescript ACT Legislative Assembly on the north side of Lake Burley Griffin, and the federal Parliament on the southside’s Capital Hill, lies the High Court of Australia. It is here that the ACT and federal governments will fight over marriage equality.

Last week federal Attorney-General George Brandis took the unusual step of announcing an intention to challenge legislation that had not yet been passed: the ACT’s proposed same-sex marriage laws. Brandis’ announcement was the latest shot taken in a Territory / Commonwealth battle that began with the ACT’s declaration that it would move on marriage equality, less than a week after September’s federal election.

The case will not reach the High Court until the Legislative Assembly has passed the laws; that’s likely to happen within the next few months. So while the precise terms of any Commonwealth challenge are uncertain, the key issue is likely to be this: is the ACT’s marriage equality law “inconsistent” with the Commonwealth’s Marriage Act? At issue is whether the ACT’s law would be capable of operating concurrently with Commonwealth law.

Here’s the key legislation. The constitution allows the Commonwealth to pass laws on “marriage” and “divorce”. The Commonwealth’s Marriage Act was enacted in 1961, and in 2004 the Howard government amended it to state that “marriage … means the union of a man and a woman to the exclusion of all others, voluntarily entered into for life”. In the current draft of its marriage equality law, the ACT hopes to avoid any claim of inconsistency by making it clear that its law applies only to those who could not get married under the Marriage Act. “You keep your law about heterosexual marriage,” the ACT seems to be saying, “and we’ll keep ours about same-sex marriage”.

In taking this approach, the ACT government is relying on legal advice received several years ago regarding the debate over civil unions, which it believes applies to the debate over marriage equality. The advice indicates that the ACT’s laws would be valid, and suggests the ACT may have greater latitude to legislate on marriage equality than the states.

But the federal government has its own advice. That advice has not been released, but seems to indicate the ACT’s laws would be invalid; that is, inconsistent with Commonwealth law. The Commonwealth may argue that its Marriage Act must effectively be seen as the definitive word on all marriage law in Australia, and that legal chaos would ensue if two marriage regimes operated side-by-side for different types of couples.

It is difficult to predict what view the High Court would take. It may come down to technical arguments about the nature of “inconsistency”. Early reports suggest some conservative state governments are lining up behind the Commonwealth in its mooted challenge to the ACT laws. Some may be surprised that the Queensland LNP government, so often one to emphasise states’ rights over the rights of the national government, may choose to back the federal government.

If the High Court’s ruling is narrowly confined to the extent of Territory legislative power, then this may have only limited consequences for the states. But if the High Court’s judgment makes wider points about the nature of contemporary federalism, Queensland may wonder if it has backed the right constitutional horse.

Realpolitik may, to some extent, overtake events in the courts. Brandis has expressed concern that, if the ACT activates its marriage laws before there is a result in the High Court, couples married under the laws may be forced to live with a cloud over the legitimacy of their marriage while waiting for the ruling. Brandis asked the ACT to delay activating the laws. ACT Chief Minister Katy Gallagher announced via Twitter that the ACT would not obey. Australian Marriage Equality expects thousands of same-sex couples from across Australia to travel to Canberra for weddings under the Territory’s marriage equality laws. Many of these couples may want to walk down the aisle under the ACT laws even if the High Court had not yet handed down a judgment.

This is not just a fight about marriage equality. It is also a fight about territory rights and the rights of the people who live there. Interestingly, the Commonwealth government chose to challenge the ACT laws in the High Court, rather than by passing its own law. The existence of the latter option, which is one that cannot be taken with respect to the states, is seen by many in the territories as relegating them to second-class citizens with lesser democratic rights than those in neighbouring states. While indicating his intention to fight the Commonwealth’s High Court challenge, ACT Attorney-General Simon Corbell welcomed the fact that the challenge was being brought to a court, rather than federal Parliament, insofar as this would treat the ACT’s laws similarly to those of a state.

The Canberran battle over marriage equality is just getting started.