Australia just had its largest ever rally for marriage equality.

With AAP. 

A record number of LGBTIQ supporters have filled central Sydney in support of same-sex marriage.

Tens of thousands of people, including federal Opposition Leader Bill Shorten, deputy Tanya Plibersek and a “rainbow coalition of politics”, crowded around Town Hall on Sunday afternoon.

“We’ve got one last mountain to climb before we make marriage equality a reality, let’s climb it together today,” Mr Shorten told the rainbow-clad crowd.

He apologised to LGBTIQ Australians for “all of the hateful and stupid things which have been said and are going to be said until we win marriage equality”.

Promising to call out the rubbish and the hate, Mr Shorten urged those left disappointed by the High Court’s decision to allow a postal survey to “turn your disappointment into determination to win.”

NSW Greens MP Jenny Leong said a rainbow-painted Sydney has come out to show love will win.

"There is no choice now but to get out the most enormously massive rainbow 'Yes' vote that we can possibly muster," she said.

The colourful crowd waved signs such as "Let's end the hate", "Did I vote on your marriage?" and "How is this not a thing yet?"

To many people's surprise, Prime Minister Malcolm Turnbull also made an appearance at the rally, throwing his support behind the "yes" campaign.

Organisers said it was the biggest demonstration for LGBTIQ people in Australia's history, which left Town Hall train station at a standstill, filled to capacity.

The masses marched down Park Street and Elizabeth Street towards Circular Quay where they were met by comedian Pauline Pantsdown.

Speaking before the rally, she said she was there to provide balance, representing the "No the whole thing is wrong and it stinks and I don't like it campaign".

"I can't think of any valid reasons against two people loving each other or being together, having support, having community around them," she said.

"I will by the time the rally starts come up with some completely random, unrelated reasons and I'll see you all then."

The Australian Bureau of Statistics will start mailing out survey forms on Tuesday, with results announced on November 15.