Australia’s leading constitutional barrister, Bret Walker SC, has cleared the way for states to legislate for same-sex marriage in a landmark legal opinion released on Friday which says the Tasmanian Same-Sex Marriage Bill is constitutionally valid.

But there was a sting in the tail of the Walker advice for the ACT because its same-sex marriage legislation, which passed today, was originally framed differently from Tasmania’s and, in Walker’s eyes, was doomed to fail any High Court challenge.

The advice — solicited by the Human Rights Law Centre acting on behalf of Australian Marriage Equality and co-authored by two other prominent constitutional barristers, Chris Young and Perry Herzfeld — is a game-changer in the debate on state same-sex marriage laws.

Walker is widely respected; his opinion is significant because concerns about the constitutionality of state same-sex marriage laws have been one of the main obstacles to passing them.

For example, the Tasmanian Same-Sex Marriage Bill was defeated by two votes last year and several of those who voted against on constitutional grounds have previously lauded Walker.

Upper House members who are key to passing the Same-Sex Marriage Bill have praised Walker as “Australia's acknowledged leading constitutional lawyer”, “the country's top constitutional law expert”, and “one of the very best in Australia”.

Clearly, if anything can assuage the concerns of sceptical MPs in Tasmania, NSW and elsewhere, and convince them to vote for a state bill, it is the Walker advice.

We will know soon enough, with a motion to re-introduce the Tasmanian Bill due to be debated in less than two weeks and debate on the NSW Bill due in that state’s Upper House not long after.

The Walker advice isn’t the first foray into this area. Australia’s leading constitutional academic, George Williams, was the first to raise the idea that states can legitimately pass laws for same-sex marriages.

In recent months two reports addressed the constitutionality of state same-sex marriage laws – one from the NSW Parliament and one from the Tasmanian Law Reform Institute.

Both argued convincingly that the states have the power to enact laws for same-sex marriages and that the case for the constitutional validity in the High Court would be a strong one.

Where the Walker advice differs from previous contributions to the state marriage debate is its analysis of the specific way state statutes are framed.

According to Walker and his co-authors the Tasmanian Same-Sex Marriage Bill is constitutionally valid because it has been carefully framed to avoid any conflict with the Federal Marriage Act.

Rather than seeking to allow same-sex couples to marry, it allows couples to enter same-sex marriages.

That may seem like hair splitting, but it matters because in Walker’s view the Commonwealth Marriage Act has conclusively defined who can and who can’t enter the legal institution called “marriage”.

But it has not defined who can enter the legal institution called “same-sex marriage”. In this field the states are free to do what they wish.

Of course, there will be some people who object to the idea of same-sex marriage being a legal status distinct from marriage.

But as Walker explains, the distinction is no greater than the distinction between, say, marriage and de facto marriage. Federal marriages and state same-sex marriages retain the same attributes.

State same-sex marriage laws allow same-sex couples to enter into fully fledged, legally binding, exclusive and life-long unions with the name “marriage”.

In this respect it’s worth noting, as the Tasmanian Law Reform Institute did in its report, that marriage equality in the UK was achieved through a separate law about same-sex marriage rather than the amendment of an existing marriage statute.

Lawyers may argue about definitions but I believe most couples ultimately don’t care how the law they marry under is framed. What they care about is that they can marry, and that their marriage will still be valid in a year’s time, which the Tasmanian and NSW Bills promise they will be.

This was not a promise the ACT Bill initially made. The law first proposed by the ACT Government did exactly what Walker says it couldn’t do — it trespassed on the federal government’s territory by expanding the existing legal status of “marriage” to include same-sex couples.

Williams and others also expressed concern about this constitutional problem. The ACT Government had the best motives. Like all marriage equality supporters it wants there to be no legal distinction between the marriages of gay and straight couples.

But according to the experts, this is simply not possible if the states and territories are to move the issue forward in the face of an unfriendly federal government.

The ACT moved to strengthen its Bill, responding to expert advice by announcing that it will make its Bill more like bills in Tasmania and NSW.

Another problem with the original ACT Bill was the way it opened marriage, not just to same-sex couples, but to anyone who can’t currently marry. This is a hat-tip to transgender and intersex people who don’t identify as male or female and don’t want to marry under a gender specific statute.

Again, I understand the intention. But the outcome may have been a potential constitutional conflict when a couple marries under the territory law who are legally male and female in federal law regardless of how they might identify in their own hearts. Unfortunately, no state legislation can change the way the federal government recognises gender.

Besides, contrary to some reports, no-one actually misses out on the chance to marry under the Tasmanian model, or under the new ACT model.

In these jurisdictions, all transgender and intersex people are legally male or female, meaning in practise those who can’t marry under the federal Marriage Act will be able to marry under a state or territory law specific to same-sex couples.

The interest most of my transgender friends have in this debate is to legally marry, not to legally redefine gender, and a state same-sex marriage statute is currently the best chance they have of achieving their goal.

When same-sex couples begin to marry soon in the ACT it will be a joyous day for supporters of equality across the nation. It will change the debate forever when same-sex couples begin to wed without the sky falling in.

What will make it even better is knowing these couples’ solemn vows of lifelong commitment have the best possible chance of withstanding a challenge to their validity in the High Court.