The postal survey saw 16 million registered voters among Australia's population of 24 million asked for their views on overturning the prohibition.
With a turnout of 79.5%, the vote had a bigger turnout than the Brexit referendum and US presidential election.
As the majority voted in favour, the parliament will vote by December on legislation to lift the prohibition.
The poll is non-binding but Prime Minister Malcolm Turnbull immediately said he would fulfill a pledge to put a proposal to parliament to pass laws on marriage equality by the end of the year.
"It is unequivocal, it is overwhelming," Turnbull said of the survey result.
"They have spoken in their millions and they have voted overwhelmingly yes for marriage equality," he told reporters in Canberra. "They voted yes for fairness, yes for commitment, yes for love."
The result marks a watershed moment for gay rights in Australia, where it was illegal in some states to engage in homosexual activity until 1997.
Thousands of supporters who had gathered in a central Sydney park to mark the result broke into a loud cheer, hugged each other and cried as it was delivered live over a big screen by the country's chief statistician.
The result boosted the carnival atmosphere with some people carrying banners declaring "our love is real" and many sheltering from the hot sun under rainbow umbrellas. Among the crowd were men and women wearing wedding dresses and corsets and sequined suits.