Bill Shorten has urged supporters of same-sex marriage to get behind the "yes" case, despite still holding concerns about the government's postal ballot.
The opposition leader has rejected calls for a boycott of the $122 million survey to be conducted by the Australian Bureau of Statistics and instead issued a rallying call in parliament on Thursday.
Despite opponents of the law change such as Tony Abbott wanting the poll to fail, Mr Shorten said the most powerful act of resistance was to vote "yes for equality".
"I will be voting yes. I will be campaigning for a yes vote," he said.
Two High Court challenges to the ballot have been launched, claiming it breaches the constitution and the proper legal basis is not in force to fund it.
However, the government is confident it has the legal and constitutional power for it to go ahead.
If the court challenges are successful, law change advocates won't be getting any joy from Prime Minister Malcolm Turnbull.
He says no private member's bill will go to parliament without giving the Australian people a say, as promised at the 2016 election.
Mr Turnbull has also said he will personally vote "yes" and advocate a "yes" vote, but won't be spending a lot of his time campaigning on it.
The ABS has begun work on the voluntary poll - which it calls a "postal survey" - with forms due to be posted by September 12, completed by November 7 and a result to be announced on November 15.
Voters have until the close of business on August 24 to update their enrolment details, or enrol for the first time.