Australian Attorney-General Nicola Roxon said she would vote for gay marriage when the chance came later this year and the "time is coming" for change, as protesters marched across the nation.
But Roxon also stood behind Prime Minister Julia Gillard, who stridently opposes legalising same-sex marriages, saying "people of good will can have different views on it."
The attorney-general was speaking as thousands of Australians held protest marches across the country on Saturday, calling on Gillard to change her stance after US President Barack Obama said that same sex couples should be able to wed.
Australia passed an amendment to its laws in 2004 explicitly defining marriage as between a man and woman, but there are several bills before the parliament calling for the right to be extended to same-sex couples.
Activists believe pressure is mounting for Labor to push for change, with the party changing its official stance to pro-gay marriage last December after 10,000 protesters marched on its national policy summit.
Labor also decided that MPs should be allowed to vote with their conscience rather than along party lines and Roxon said: "I've made clear that I will be voting in favour of same-sex marriage."
"People have different views and people of -- you know, good will can have different views on a matter like this but I've made my decision clear," Roxon told reporters.
She added: "I think that it's an issue that's time is coming and I think many people -- me included -- have, over time, been persuaded of the importance of this."
Gillard said this week she would not vote in support of gay marriage despite Obama's admission that he was in favour, describing it as a "deeply personal question," and Roxon said individuals were entitled to their views.
Because the conservative Liberal/National opposition has refused to allow its own MPs a conscience vote and will reject the legislation as a bloc there is little prospect of the laws changing.
Every Labor lawmaker and a majority of independent MPs would have to vote in favour to ensure its passage -- an unlikely prospect on such a divisive issue.
In Australia marriage is mandated by federal legislation, so although civil same-sex unions are recognised in five states, the couples are not seen as "married" by the national government.
All the same, same-sex couples have the same rights as heterosexual couples in areas such as pension schemes and medical benefits.