Opinion polls throughout the vote have put the Yes campaign ahead, which supports a change in the law.
About three quarters of the electorate - more than 12.6 million people - took part in the eight week-postal survey.
Prime Minister Malcolm Turnbull supports marriage equality but faces conservative opposition within his governing coalition.
He has pledged that if there is a Yes vote in the non-binding poll, he will take steps to change marriage laws.
"We won't stop working until every couple is equal under the law," the Equality Campaign, the group at the forefront of the Yes vote, said in a statement on Tuesday.
"If the result is Yes, our politicians will need to follow through on a Yes result by passing a fair bill that supports true equality."
The No campaign, with the support of religious groups, has put the focus on the definition of family, raising concerns about how issues like gender will be taught in schools.
The survey has been voluntary, unlike Australia's compulsory elections.
Each side has accused the other of bullying and misleading discussion, while the debate has even been linked to violent incidents.
Photo copyright: AFP. Caption: The Yes campaign is confident of victory