Same-sex marriage advocates say having the backing of deputy Liberal leader Julie Bishop will be "significant" in the push for equality.
Ms Bishop told Channel Ten's The Project on Monday night she had no concerns about granting same-sex couples the right to marry and backed a proposed national plebiscite on the issue.
"I have absolutely no concerns about it myself, but I know there a lot of people who are deeply concerned about the issue," Ms Bishop said.
She said she supported Australians having their say on "such a fundamental issue" through a plebiscite.
The foreign minister has previously declined to comment on her personal views on same-sex marriage.
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Australian Marriage Equality's Rodney Croome said her support was "quite significant", given her influence and popularity.
"It will mean other members of the Liberal Party will be more likely to vote in favour, and I think if it's put to the people, then we'll see more Australians in support of the reform because of the support of Ms Bishop," he told ABC radio on Tuesday.
