A gay rights group has been granted leave to take part in a High Court challenge to the ACT's same sex marriage laws.
Australian Marriage Equality Inc applied and was granted leave to appear in the case, being fought between the Federal and territory governments.
The application was not opposed by either party.
The group will now act as an 'Amicus Curiae', or friend of the court, and provide information that bears on the case, including legal opinion or testimony.
The deputy director of Australian Marriage Equality, Ivan Hinton, has become one of the high-profile faces of the same-sex marriage movement, as he plans to marry his partner Chris Teoh on Saturday.
The couple are already married under Canadian law but have been part of the decade-long fight to bring marriage equality to Australia.
Their family and friends are flying into Canberra on Tuesday, hoping that a landmark case against the ACT's same-sex marriage law won't prevent the two men from marrying.
"Today I'm just a guy, I'm just one man who wants to marry the person he loves with all his heart in his home town of Canberra," Mr Hinton said.
"We sincerely hope that we will be able to celebrate that moment, our relationship and our lifelong commitment to each other."
The High Court's two-day hearing of the Commonwealth's case against the ACT government's same-sex marriage law has begun.
The landmark case before the full High Court bench is expected to decide whether or not the ACT has enacted a law that is in conflict with the federal marriage act and the federal family law act.
The case is the first real test of whether or not states and territories can legislate for same-sex marriage.
Attorney-General George Brandis has repeatedly refused to comment in the lead-up to the hearing, but has previously described the ACT's law as a "threat" to the "well-established position" that marriage laws should be nationally uniform.
The Abbott government acted swiftly to strike down the ACT law and mounted its challenge immediately after the Legislative Assembly passed the same-sex marriage bill in October.
The ACT government faced criticism of its bill, which advocates and constitutional law experts claimed had been drafted too quickly and did not go far enough to create a separate status of same-sex marriage to limit the chance it would be overturned by the High Court.
But the ACT government has backed its bill as being constitutionally sound and capable of concurrent operation with federal marriage laws.
ACT Attorney-General Simon Corbell said the government was apprehensive about Tuesday and Wednesday's hearing but was confident it had "strong arguments to put to the court".
The ACT expects the High Court will rule this week, before the first marriages can take place on December 7.
The government has 47 registrations for couples who wish to marry in the ACT.
ACT Chief Minister Katy Gallagher attacked the Abbott government earlier this week for behaving as though it had "a clear cut case".
"I certainly think that the attitude of the Commonwealth that is a clear-cut case is not supported," Ms Gallagher said.
"It's certainly not supported by the legal advice of the ACT government."
Ms Gallagher said the outcome was impossible to predict.
The federal government, in launching its challenge, said it had moved quickly to avoid causing "distress" for couples who could see their marriages invalidated.
Australian Marriage Equality is seeking leave on Tuesday morning to appear in the case.
Director Rodney Croome told reporters outside the court: "Today is unprecedented and a very important day in the decade-long campaign to achieve marriage equality in this country".
"We're also seeking leave to appear because, of course, ultimately this issue is not just about the constitution," he said.
"It's about the aspirations of same-sex couples."
This week's hearing is expected to have implications for other state governments, namely New South Wales and Tasmania, that have tried to introduce similar laws.