The historic reforms were given royal assent on December 8, the final step in a process that began with a national postal vote in September.
There was supposed to be a 30-day waiting period after couples registered to marry, with the first weddings expected on January 9.
But some sought an exemption due to their circumstances, and the first ceremonies went ahead on Saturday, one in Sydney and one in Melbourne.
"Australia's first legal same-sex marriage has just taken place. Congratulations Amy and Lauren," Equal Marriage Rights Australia announced on its Facebook page.
Fairfax Media said Lauren Price, 31, and Amy Laker, 29, solemnised their vows in Sydney, while Melbourne couple Amy and Elise McDonald -- who coincidentally have the same surname -- tied the knot two hours later.
A photo showed Price and Laker dressed in traditional white gowns and holding hands as they said "I do" on a glorious Sydney day.
Among hundreds of comments on the Facebook page, Paul Antoine said: "Now... has the country collapsed? NOOO!! Two women have just shown their love. Pretty simple, but nonetheless special in my view!"
Jason Chapman added: "Relentless dedication from so many over the last fifty years have led us to enjoy this beautiful moment of equality. Congratulations ladies."
All but four members of Australia's 150-seat lower House of Representatives voted in support of marriage equality, with the legislation passing without any religious freedom amendments that some were pushing for.
The bill was introduced in parliament by the conservative government after Australians in November endorsed the reforms in a contentious voluntary postal vote.
Nearly 80 percent of eligible voters took part in the poll, and almost 62 percent of those who voted chose "yes" on their ballots.
Fairfax said Price and Laker had originally intended to hold a commitment ceremony at the British consulate because Price was from Wales.
But when the new law was passed, they sought an exemption from the normal waiting period because Price's family had already travelled to Sydney for the celebrations.
Melbourne's Amy and Elise McDonald, aged 36 and 28, reportedly only got permission to marry on Saturday morning, gaining an exemption for similar reasons, according to the Herald Sun newspaper.
They were allowed to have a legal marriage on financial grounds and because their families had travelled from overseas.
Same-sex marriage is now recognised in more than 20 countries, of which 16 are in Europe.