ACT Chief Minister Katy Gallagher says her Government will be pushing ahead with same-sex marriage laws despite opposition from the Federal Government.
The ACT Labor Government expects the law to pass with the support of Greens MLA Shane Rattenbury when the Legislative Assembly sits later this month.
If so, the first same-sex marriages could happen as early as December.
But Federal Attorney-General George Brandis says the Commonwealth will be challenging the proposed laws in the High Court.
Ms Gallagher says she received a phone call from Senator Brandis asking her to scrap the laws, but she refused.
"He gave me a courtesy call yesterday (Wednesday) afternoon to say that he'd received advice," she told Lateline last night.
"He asked me not to commence the laws once they'd passed.
"We politely refused and said we would be commencing them and we'd see him in court."
Senator Brandis says he has advice the ACT's Marriage Equality Bill is "invalid by reason of inconsistency" with the Commonwealth Marriage Act.
Same-sex marriage is legal in:
- New Zealand, 2013
- Uruguay, 2013
- Denmark, 2012
- Argentina, 2010
- Portugal, 2010
- Iceland, 2010
- Sweden, 2009
- Norway, 2009
- South Africa, 2006
- Spain, 2005
- Canada, 2005
- Belgium, 2003
- The Netherlands, 2000
"Irrespective of anyone's views on the desirability or otherwise of same-sex marriage, it is clearly in Australia's interests that there be nationally consistent marriage laws," Senator Brandis said in a statement.
"At the moment, the Commonwealth Marriage Act provides that consistency.
"The ACT's proposed law is a threat to that well-established position."
Prime Minister Tony Abbott says the legal challenge is about upholding the Constitution.
"It is pretty clear under our Constitution that it is the Commonwealth which has responsibility for the rules regarding marriage," he said.
"We think it's important that there be a uniform approach to marriage throughout the Commonwealth. That's what we're going to do our best to ensure."
Incoming ACT Liberal Senator and former ACT opposition leader Zed Seselja says it is clear the Commonwealth has the power over marriage laws and the Territory does not.
"I don't support same-sex marriage. I believe that same-sex marriage or marriage generally is the preserve of the Federal Parliament," he said.
"So whether your views are in favour of same-sex marriage or against same-sex marriage, the correct place for this to be debated is in the Commonwealth Parliament."
But Ms Gallagher says the Territory Government has received different advice.
"We think [the ACT laws] can operate concurrently with the Federal Marriage Act, that's our clear advice," she said.
"Section 51 of the Constitution doesn't provide exclusive powers for the Commonwealth in this area.
"Our law shouldn't be inconsistent with the Commonwealth law, we have drafted our laws very carefully to avoid that, to allow them to run concurrently, and that's the position we will be taking to the High Court."
Ms Gallagher says the Territory is not trying to bring on a fight with the Federal Government.
"We're not seeking to bring on a fight with the Commonwealth," she said.
"We wanted the Federal Parliament to deal with this issue and to resolve it.
"They've failed to do so.
"Our bill sat there while that debate was underway in the Federal Parliament.
"It didn't deal with that issue, so we are now returning to it."
ACT Attorney-General Simon Corbell says his Government will not give in to what he sees as political bullying.
"We have expert legal opinion ourselves, from distinguished constitutional experts such as professor George Williams from the University of New South Wales," he told AM.
"Now for the first time, the High Court will be asked to adjudicate on this question and we will vigorously and robustly put our arguments to the court on that question."
Professor George Williams says it will be a battle fought over federalism.
"The [judges] will be thinking particularly about how much room a territory or state should have to engage in social policy changes of this kind," he said.
"On the other hand some judges will be convinced perhaps about the need to have stronger control by the Federal Government.
"It really is an open question that will take up a lot of time, a lot of argument and is one that will be studied for years afterwards."
Greens leader Christine Milne expects there to be a strong reaction within the gay community once the national capital enacts the law.
"I think what will happen is it will be marriage season in the ACT and people will go and get married, and then they will join the ACT in the High Court challenge," she said.
Shorten wants to see Government's legal advice
Federal Labor leadership candidate Bill Shorten says he wants to see the Commonwealth's legal advice on the matter.
"I do respect the ability of states and territories where they have the power to exercise power, that's the nature of our Constitution," he told ABC News Breakfast.
"I think we're going to have to see a little bit more from why the Federal Government is contesting the constitutional argument.
"I do support, in a federation, the ability of Governments at the state and territory level to make rules to the extent that they're allowed to legally and we'll see where this argument goes.
"It's a matter of record.
"I voted for same-sex marriage when the matter came before the Federal Parliament, but I don't think this issue is just about this."
Federal Labor MP Stephen Jones says the time has come for same-sex marriage to become law.
"It's a very disappointing decision by George Brandis," he said.
"Instead of referring this bill to the High Court, he should refer it to the Parliament.
"Tony Abbott and George Brandis should be giving their MPs a conscience vote on this.
"Give them the opportunity to vote in the way that the majority of Australians want us to vote."
Why can states/territories legislate on marriage?
- Marriage laws are typically the responsibility of the Commonwealth.
- The Federal Government argues it has sole responsibility for marriage laws, but the states and territories dispute that using a loophole.
- The states and territories believe because the Commonwealth legislates specifically for marriage between a man and a women, states can then fill the gap and legislate on same-sex marriage because it is not currently covered by the Commonwealth.
- A NSW Parliamentary inquiry this year found states and territories had the power to legislate for same-sex marriage.
- This issue is what a High Court challenge will seek to answer.