Ballots will be mailed out from September 12, with a result expected some time in November.
Organizers said the gathering was Australia's largest gay rights demonstration, as a diverse range of people clad in rainbow colors converged on the heart of the city to insist on equal rights.
Sporting bodies across Australia have thrown their weight behind gay marriage, with cricket on Tuesday the latest to add its support for same-sex unions as a contentious postal vote on the issue began.
But the government refuses to release a draft bill until after the survey decides whether the Parliament will consider change. "Let's climb it together today".
Copley and Eggins have been attending marriage equality rallies in the country for years, but now with millions of Australians about to vote their opinion on the issue, this past weekend's rally in Brisbane came with a greater sense of significance.
Two past conservative prime ministers, Tony Abbott and John Howard, have publicly opposed the change.
"All I want is the right to marry Chris Hemsworth", he wrote on a placard held at a marriage equality rally.
The coalition, the lead campaigner against same-sex marriage, did not respond to requests for comment.
So far, 15 sports organisations have signed an open letter urging marriage equality, including Football Federation Australia, Basketball Australia, the National Rugby League and several Aussie Rules clubs. It includes the Australian Christian Lobby and the Greek Orthodox Archdiocese of Australia.
"We want this process to be fair and for Australians to get the opportunity to have their say in an appropriate environment", Senator Cormann said.
"Cricket must be a welcoming environment for each and every one of us, regardless of gender, cultural heritage and - importantly in the current environment - sexuality", he said.
But the success of the "yes" vote is not certain, with a poll conducted for a major same-sex marriage advocacy group and shared with Fairfax Media on Saturday showing that support for the change was falling.