The federal government is considering whether it should block the ACT's proposed same-sex marriage legislation.
The ACT government introduced a bill to permit same-sex marriages into the Legislative Assembly on Thursday morning. It is expected to pass the 17-member Assembly with the support of all eight Labor and the only Greens member.
Same-sex marriage challenge?
Don't silence Hanson, debate her
Tanya Plibersek's new job
Linda Burney on Labor's front bench
Brandis: Terror knowledge 'constantly reviewed'
Australia 'resilient' in wake of Brexit
Rudd for UN boss?
Liberal MP's ambitious plan
Same-sex marriage challenge?
Tony Abbott has sought advice from Attorney-General George Brandis on the legality of ACT same-sex marriage legislation. Nine news.
Mr Abbott later said the Commonwealth had constitutional responsibility for marriage and Attorney-General George Brandis would seek advice on the ACT bill.
"The ACT is entitled to do what it can within the law," Mr Abbott said.
"And, as you know, under the constitution the Commonwealth has responsibility for marriage. The attorney will be seeking advice on precisely how far that extends."
The Federal Parliament has the power to disallow territory laws.
ACT Chief Minister Katy Gallagher said her government believed the same-sex marriage bill was constitutional.
"The ACT sought legal advice in 2008 which shows that a bill that did not contradict the Federal Marriage Act would be constitutionally valid," Ms Gallagher said.
"If the bill is passed, same-sex couples will be able to marry under the law and therefore can work alongside the Commonwealth Marriage Act.
"The ACT believes that we have the ability to legislate."
The Prime Minister is a strident opponent of gay marriage and, unlike the situation within the Labor Party, has not allowed his Liberal and Nationals colleagues a conscience vote on the issue.
This is despite many Liberals, including Communications Minister Malcolm Turnbull, parliamentary secretary Simon Birmingham and prominent MP Kelly O'Dwyer, supporting the legalisation of gay marriage.
If Mr Abbott wants to overturn the ACT legislation there are two ways he can do it, says constitutional law expert George Williams.
The first would be to mount a challenge through the High Court. Though Professor Williams said the Prime Minister might find this difficult given the ACT same-sex marriage bill would have been drafted with a view to withstanding a High Court challenge.
The second way to override the ACT government's sanctioning of gay marriage would be for Mr Abbott to get a bill through both houses of Parliament that specifically refutes the legislation. The Howard government took similar action in 1997 when it overturned the Northern Territory's voluntary euthanasia law, Professor Williams said. That effort was led by current Social Services Minister Kevin Andrews.
"The Commonwealth has got the power [to override the ACT law]," Professor Williams said. "The question is have they got the numbers?"
Greens Senator Sarah Hanson-Young is urging the Labor Party to commit to voting as a bloc to help the Greens stymie any attempt by Mr Abbott to override the ACT same-sex marriage bill.
Fairfax Media has contacted both Labor leadership contenders, Bill Shorten and Anthony Albanese, to see whether they would agree to block Mr Abbott.
Earlier, ACT Attorney-General Simon Corbell introduced the same-sex marriage bill into the Assembly.
"It is a bill that says people in a same-sex relationship are able to have their love and commitment to each other legally recognised in the same way that people in a heterosexual relationship are able to through a legally recognised marriage," Mr Corbell said.
Ministers of religion would not be required to officiate over same-sex weddings. Any divorce cases would be heard in the ACT Supreme Court.
Mr Corbell received a standing ovation from the public gallery after the debate was adjourned.
The ACT's Deputy Chief Minister, Andrew Barr, who is gay and has a long-term partner, said he fought back tears while listening to Mr Corbell endorse the bill.
"We're doing I think a really good thing today. It speaks to our values, it speaks to why we're in politics," Mr Barr said.
Opposition Leader Jeremy Hanson said the Canberra Liberals would not support the legislation when it is debated by the Assembly next month.
"Firstly, it's likely to be found unconstitutional and certainly I think everybody agrees it's going to be subject to a High Court challenge," he said.
"Secondly, it's just simply not the job of the ACT Assembly to be determining what is clearly controversial, national social reform.
"For those two reasons we won't be supporting it.
"Within the Liberal Party, there are a range of views about same-sex marriage but what we all agree on is it shouldn't be debated by the Legislative Assembly of the ACT, which is Australia's smallest jurisdiction."