Same-sex couples have a small window of opportunity to marry under ACT law, with the High Court due to rule on the validity of the legislation five days after it comes into effect.
The Commonwealth has challenged the ACT's Marriage Equality Bill, which passed in October, arguing that marriage is governed solely by federal law and that "uniform and exhaustive" national regulation should remain.
"If you ask a question 'who can a person marry?' the sole answer is in the federal law," Australian Solicitor-General Justin Gleeson, SC, told the court on Tuesday.
"A man and a woman can marry ... you cannot marry a person under age or if a person is in a previous state of marriage."
He said the Marriage Act is based on important elements of Australian Federation and couples bound by it have access to "matrimonial clauses", including divorce.
"(The ACT law) is not to be construed to be so extensive to allow the Territory to impose laws which alter, impair or detract from federal law," Mr Gleeson said.
Such a law would be "repugnant to the grandeur of power", he added.
But ACT Solicitor-General Peter Garrisson, SC, defended the Territory bill and said it can sit "alongside" federal law without detriment.
"It doesn't touch with the Marriage Act as it currently stands because that covers a particular form of relationship (between a man and a woman)," he said.
While the Marriage Act excludes the union of people on the grounds of age and bigamy, it does not rule out same-sex couples, said human rights lawyer Kate Eastman, SC, acting for the ACT.
"For certainty, one would expect to see in the category of relationships that would give rise to a void relationship, a union between a man and a man or a woman and woman," Ms Eastman said.
The lack of such prohibition means the Commonwealth has not legislated to prevent states and territories introducing laws parallel to the Marriage Act, she added.
After hearing a day of submissions, the court's full bench reserved its judgment until December 12.
The announcement was welcome news for Australian Marriage Equality member Ivan Hinton who anxiously watched the hearing ahead of a Saturday ceremony when he will marry his partner of 11 years, Chris Teoh.
The new laws require a "notice of intent" to be lodged by couples at least a month before a ceremony overseen by an accredited celebrant.
The earliest possible date for ceremonies is Saturday, when Mr Hinton said he knows of at least 12 same-sex couples planning to marry in Canberra.
"I didn't want to let Saturday pass without celebrating our relationship," he said.
If the court overturns the ACT law, it is unclear what status will be afforded to couples who have already married.
