Prime Minister Malcolm Turnbull has urged Australians to say yes in the postal survey as thousands packed out Sydney's CBD to rally for same-sex marriage.

Thousands gathered for a march from Town Hall to Hyde Park in support of a change to the Marriage Act, with the gathering causing significant crowd congestion in the city centre.

Organisers have declared the rally the largest LGBTI demonstration in Australian history as thousands more took to Brisbane streets to show their support.

Crowds have filled the CBD in front of Town Hall to rally for same-sex marriage. ( ABC News: Jean Kennedy )

Speaking earlier at the NSW Liberals and Nationals for YES campaign launch in Sydney, Mr Turnbull said the question facing the nation was one of fairness.

"Throughout my public life I've sought to ensure same-sex couples are not discriminated against and their entitlements, be it in respect of medical benefits, taxation, superannuation or employment, are no different to those afforded to heterosexual couples," he said.

"Why then shouldn't those same rights now be extended to marriage?"

Thousands gathered at Sydney's Town Hall to show their support for same-sex marriage. ( AAP: Paul Miller )

Mr Turnbull pointed to the fact that same-sex marriage had already been legislated for in 23 countries, including several with which Australia has close cultural affinities.

"In any one of those nations, has the sky fallen in? Has life as we know it come to a halt? Has traditional marriage as we know it been undermined? The answer is no."

Those protesting had many colourful and creative placards in hand. ( ABC News: David Spicer )

Mr Turnbull also asked Australians to "be honest with each other" and see that the only threat to marriage is a lack of loving commitment, not gay couples.

"I am utterly unpersuaded by the proposition that my marriage to Lucy, 38 years long next March, or indeed any marriage, is undermined by two gay men or two gay women setting up house down the road, whether it is called a marriage or not."

Crowd causes congestion in central Sydney

Sorry, this video has expired Opposition Leader Bill Shorten addresses same-sex marriage supporters in Sydney.

Opposition Leader Bill Shorten, speaking at this afternoon's rally at Sydney's Town Hall, gave credit to Mr Turnbull for supporting the Yes cause but said it is not politicians who will make the campaign a success.

"If we win this survey, it won't be because of a particular politician or politicians," he said on Sunday.

"It'll be because Australia has said it's time to make marriage equality a reality.

"I'm very optimistic, and today only improves my sense of possibility."

Sydney hosted a large rally of same-sex marriage supporters on Sunday. ( ABC News: Jean Kennedy )

Sydney's Deputy Lord Mayor Kerryn Phelps said she wants to see all Australians united in supporting the yes case.

"This is an important time for Australia as a nation and the yes campaign is all about uniting our nation... we don't want the negativity or the division of the no campaign," she said.

The rally comes just days before the Australian Bureau of Statistics is due to send out the survey forms.