The proposed Tasmanian legislation is a departure from Ms Gillard's vocal political and personal opposition to same-sex marriage and yesterday she made it clear she considered gay marriage to be a federal concern.

''The Marriage Act is a federal law and we do have a bill before the federal Parliament dealing with same-sex marriage,'' the Prime Minister told reporters in Cairns. ''I determined that this should be a conscience vote for the Labor Party and people will be free to determine how they vote.''

The Greens called upon Ms Gillard to rule out a constitutional challenge to the Tasmanian legislation but she did not.

''We don't have any details on [the Tasmanian gay marriage bill], so it's far too early for anything like that,'' she said.

Ms Giddings said her advice from the Tasmanian Solicitor-General suggests the state could win a constitutional challenge.

''I'm reasonably confident it would,'' Ms Giddings told reporters at the Tasmanian ALP's state conference in Hobart. ''This is about doing what is right … there is nothing that I have received in my legal advice that would preclude the state government from pursuing this matter and legalising marriage here in Tasmania.''

The Treasurer, Wayne Swan, gave a television interview backing his leader's stance.

''Tasmania will do what it wants to do … but what we will do in Canberra is that we'll have a conscience vote on this issue in the Parliament,'' he said. ''I think that's the way to handle the issue, in the national Parliament.''

The gay marriage issue has flared up just as Labor is trying to woo back the blue collar voters it has lost to the Coalition, particularly in Queensland, where leaked internal party polling reportedly shows Labor's two-party preferred vote has plunged to 36 per cent.

Returning to work after a week's holiday in Port Douglas, Ms Gillard visited Cairns yesterday to announce the city would host the G20 finance ministers' meeting in 2014.

The meeting is part of the G20 leaders' summit, which will take place in Brisbane.

Ms Gillard said the meeting would be a boost to the Queensland tourism industry, which has been dealt a heavy blow by the high Australian dollar.

''Including media, around 2000 people will come to Cairns because of the G20 finance ministers' meeting,'' she said. ''We have wanted to benefit Cairns because we do know that it's been a bit tough in the recent period.''

Ms Gillard would not countenance a question on the security of her leadership, saying: ''I can't be bothered with any of that today.''

The Attorney-General, Nicola Roxon, has confirmed she would quit the ministry if former prime minister Kevin Rudd were to stage a comeback.

Asked during a television interview if she stood by earlier statements that she would refuse to serve in a government led by Mr Rudd, she said: ''Yes and yes … I am confident that Julia will lead us to the next election and I stand by the comments that I've made before.''

Ms Roxon said she had made her views on Mr Rudd's leadership ''very clear'' in the lead-up to the Gillard-Rudd leadership spill in February. ''I didn't enjoy making them clear,'' she said.