After years of political stagnation, the public has now tasked the Turnbull government with changing the law before Christmas to allow same-sex couples to marry.

Almost 80 per cent of eligible voters participated in the unprecedented voluntary postal survey, giving the verdict an authority unmatched by most elections globally.

It means Australia is poised to join 25 other countries that have granted marriage equality to gay couples, including the US, Canada, New Zealand and the United Kingdom.

At street parties across the country, gay and lesbian Australians cheered, danced and embraced as the results were announced by the chief statistician on Wednesday.

It is a landmark moment in Australia's mixed civil rights record: it was one of the first countries to give women the vote, but still struggles with Indigenous reconciliation and is one of the last English-speaking democracies to legalise same-sex marriage.

The result is also a significant victory for Prime Minister Malcolm Turnbull, of the ruling centre-right Coalition, who is a longstanding supporter of same-sex marriage and firmly believed the "yes" vote would prevail.

"Love has had a landslide victory," declared Alex Greenwich, co-chair of the Equality Campaign, from a public gathering in Sydney. "Getting to this point has not been easy, but rarely in your life can you celebrate with such pride overcoming adversity to make history."

Mr Greenwich said the campaign's support and momentum had exceeded all expectations, and the result had delivered "an unequivocal mandate" for politicians to vote through the change by the end of the year.