So the ACT passed a civil union law. Whether this law and any same-s-x marriage laws introduced in Tasmania can be overturned are a funny mix of politics and constitutional law.

The federal government has the power to overturn the ACT’s civil union law under the constitution. Our constitution gives the federal government the power to make and overturn laws in the ACT and NT.

This is how John Howard overturned the ACT’s last civil union law in 2006. What’s interesting this time is the politics of whether the ALP decides to intervene to overturn the laws of another Labor government, particularly given their party platform is now pro-same-s-x marriage. Definitely one to watch.

If Tasmania were to pass a same-s-x marriage law, it would be slightly more difficult for the federal government to overturn it. That’s because the Tasmanian law would need to be found to be in conflict with the Federal Marriage Act.

This would be based on whether the Federal Marriage Act regulates all marriages or merely opposite-s-x marriages. Howard tried to pre-emptively quash any state laws introducing same-s-x marriage by introducing a definition into the Marriage Act that marriage is between a man and a woman. Whether this means that the act is regulating same-s-x marriage by defining marriage to exclude same-s-x marriage will be a question for the High Court.

In any case, even if a Tasmanian same-s-x marriage law and the federal Marriage Act weren’t in direct conflict, the federal government could pass a new law forbidding same-s-x marriage to create the conflict. Such a conflict would then mean that the Tasmanian law was inoperative.

The Gillard government would be unlikely to do this given the Labor Party platform supports same-s-x marriage (although, in the current political climate anything is possible). If the Liberals were elected, introducing a law to render the Tasmanian law inoperative would probably be one of their first roll backs of fundamental rights. Fortunately though, such a law could not be retrospective so marriages would not suddenly be legally void.

The only way a Tasmanian law for gay marriage could be found to be unamendable by the federal government would be if the federal government only had a power to legislate for opposite-s-x marriage.

The ACT Liberal Senator Gary Humphries used this argument against the federal government introducing gay marriage in the Senate inquiry into marriage equality. For this to be the case, the High Court would have to find that when the federal government was given a “marriage power” in 1901 it was only to regulate what was conceived of as marriage at the time. In other words, opposite-s-x marriage.

If the High Court endorsed this interpretation of the federal government’s marriage power, it would fall to the states to regulate same-s-x marriage. This is because while the federal government must have an enumerated head of power under the constitution to make a law, the states have broad plenary power to legislate for anything and everything.

The only limitation to this is that when a state law and federal law are in conflict, the state law is ineffective.

*Hannah Lewis has just returned from having a civil union with her partner of four years in Germany. She is currently completing a masters of law (juris doctor) at Monash University and has a particular interest in the relationship between the constitution and politics. She also works part-time in community engagement.