Same-sex marriage in Australia is now, finally, legal.

The law passed on Thursday with the backing of the Federal Parliament to a packed public gallery who erupted with cheers and applause.

MPs crossed the chamber to embrace each other – some even shedding a tear.

The historic law followed last month’s postal survey, which concluded that Australia embraced marriage equality.

Thursday’s vote came after last week’s vote in the Senate, meaning the law passed both houses of parliament.

"What a day! What a day for love, for equality, for respect. Australia has done it," Prime Minister Malcolm Turnbull told parliament.

"It's time for more marriages, more commitment, more love, more respect."

Labor leader Bill Shorten said the new law spoke for a modern Australia, "inclusive and fair".

"When this law is passed, we should declare we are no longer a nation of people who voted no, or people who voted yes - we are simply Australians one and all," he said.

Both major parties had given their members a free vote on the issue.

More than 120 MPs spoke during nearly 24 hours of debate on the bill, which was sponsored by gay Liberal senator Dean Smith and backed by colleagues Warren Entsch, Trent Zimmerman, Tim Wilson and Trevor Evans.

Former prime minister Tony Abbott tried to effectively kill off the legislation with an amendment that would stop its progress, but failed.

Treasurer Scott Morrison, junior ministers Michael Sukkar and Alex Hawke, and backbenchers Andrew Hastie, Andrew Broad and Sarah Henderson, were also unsuccessful in trying to change the bill, as was Greens MP Adam Bandt.

"It's time to pop the bubbly ... because love has won," Mr Bandt said.

The legislation will become law as soon as it is granted royal assent by the Governor-General.

"Many of us older LBGTIQ people never thought we would see this day," Magda Szubanski said earlier from outside Parliament House.

"When we were young, it would have been zero percent of people who would have voted for marriage equality.

"It was inconceivable - but today, Australia, you've demonstrated you love us. We love you."

Australia is now the 26^th country to legalise same-sex marriage.

SAME-SEX MARRIAGE LAWS IN AUSTRALIA

1961: Australia introduces its first Marriage Act, which does not include a formal definition of marriage

2004: Marriage Act is amended to define marriage as a union of a man and a woman to the exclusion of all others, and bans the recognition of overseas same-sex unions

2004-15: About 20 attempts are made to introduce laws to legalise same-sex marriage to no avail

2015: Coalition government commits to a national plebiscite on same-sex marriage

2016: Senate rejects government's plebiscite plan

August 2017: Senate knocks backs plebiscite proposal again. Coalition government pursues a national voluntary postal survey instead, with ballot papers sent out the following month

November 2017: 61.6 per cent of eligible Australians vote in favour of changing the law to allow same-sex couples to marry

December 2017: Same-sex couples will be allowed to marry in Australia after new laws clear parliament.

Fairfax + AAP