The High Court could find the federal Marriage Act applies only to heterosexual couples or that new ACT same-sex laws are inconsistent with federal law.
The success or otherwise of same-sex marriage laws in the ACT could turn on whether the legislation creates a new, separate form of marriage.
The states and territories have the power to enact same-sex marriage laws, but a question remains about whether they are constitutional.
That's because, since 1961, marriage has been a shared responsibility between the commonwealth and the states.
To resolve inconsistencies, the constitution states that federal law shall prevail over state law.
Constitutional law expert George Williams, who advised the ACT government, says the High Court could find the federal Marriage Act applies only to heterosexual couples.
The ACT law seeks to create a new type of same-sex marriage under territory law rather than fill in the gaps of the Marriage Act, created by a 2004 amendment which defined marriage as "the union of a man and a woman to the exclusion of all others, voluntarily entered into for life" to ensure that the common law definition was put beyond legal challenge.
"You need to do it separately, independently, and as a result you need to set up your own special form of same-sex marriage," Williams argues.
If the ACT law is upheld, the door will open for similar laws around the nation, he says.
If it is struck down, some of the considerable momentum built up by same-sex marriage campaigners will be blunted.
The Abbott government wants the High Court to find the ACT laws are invalid by reason of inconsistency with the Marriage Act.
Liberal state governments in NSW, Queensland, Victoria and Western Australia are likely to join the Commonwealth.
The Labor-Greens government in Tasmania has told the ACT it will support its defence.
Federal Attorney-General George Brandis insists the Commonwealth is making the High Court challenge because there needs to be nationally consistent marriage laws.
Opponents argue Australia has myriad inconsistent laws across the nation.
Brandis also is concerned that same-sex couples who use the ACT law to marry risk becoming "very distressed" if the High Court later rules the law and their marriage invalid.
With the ACT government expecting the first marriage ceremonies in December, the Commonwealth will ask the High Court to hear its case as soon as possible.
ACT Attorney-General Simon Corbell believes any hearing, let alone a judgment, won't be heard until February or March next year.
That might prompt the Commonwealth to seek an injunction from the High Court, preventing the ACT from proceeding with ceremonies until its new laws are validated.