Australia's attorney-general, George Brandis, said there will be a plebiscite on legalizing same-sex marriage if the ruling conservative coalition wins a general election this year.

Senator Brandis said the bill to set up the public vote would not be introduced in this term of Parliament because Labor and the Greens would not support it, and the Senate would not have time to debate it.

Australian Prime Minister Malcolm Turnbull (right) waves to supporters as he travels with New Zealand counterpart John Key on a ferry in front of the Sydney Harbour Bridge in Sydney, Australia, on February 19, 2016.

However, opposition leader Bill Shorten, who marched with the Labour float alongside his glamorous wife Chloe, has slammed Mr Turnbull for his "half arsed" attitude towards equality.

Education Minister Simon Birmingham said the Turnbull Government wants to have the plebiscite after the next election.

"If the government allowed a free vote in Parliament we could have marriage equality next week rather than delaying until the end of the year", Mr Croome said.

Even if a plebiscite on removing restrictions in the Marriage Act that now limit marriage to between a man and woman is passed with overwhelming public support, parliamentarians will still need to vote on the issue.

Meanwhile, the Labor government is pushing for a parliamentary vote on same-sex marriage within the first 100 days of taking office.

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But he also believed there was "virtually no doubt at all" that if the public votes yes then "the parliament will follow", he said.

"This is a position I have come to over the years and after a lot of reflection", he said.

"Treating gay people equally is, I think, one of the fundamental values of modern Australian society", he said.

A spokeswoman for Mr Turnbull said only that the government was committed to holding the plebiscite "as soon after the election as can be done".

"If we were to have a plebiscite before the end of the year, and you were to reverse-engineer that, it would make interesting speculation about the timing of an election".

"The Liberal Party said that they're not going to be bound by the result ... it's just a very expensive opinion poll".

Mr Turnbull became the first prime minister to attend the Gay and Lesbian Mardi Gras on Saturday night, something Senator Brandis described as "completely unremarkable" given it is held in his inner-Sydney electorate of Wentworth and he has been several times before.