Both sides of the same-sex marriage campaign are claiming underdog status in a "David and Goliath" battle to win over 16 million Australian voters.
Forms for the voluntary postal survey will go out from Tuesday after gay rights advocates lost a High Court bid to stop the $122 million process.
Human Rights Law Centre director of legal advocacy Anna Brown says the survey is unnecessary but the advocates will now focus all their efforts on securing a resounding "yes" vote.
"We lost the court case but we need to win the plebiscite," she said.
Australian Marriage Equality co-chair and plaintiff Alex Greenwich said supporters of marriage equality now had no choice but to campaign hard for a strong yes vote.
"This is going to be a tough campaign but we are in it to win it," he said.
"But certainly the task ahead is daunting and clearly, having had this process imposed upon us in these circumstances, we are clearly the underdog."
Those pushing for a "no" vote have also claimed underdog status.
Coalition for Marriage spokesman Lyle Shelton urged prospective no-voters to donate to their campaign to ensure it can be competitive.
"It's a David and Goliath battle," the Australian Christian Lobby managing director said.
"We don't have the resources that the other side do."
Marriage Alliance CEO Damian Wyld said it was extraordinary that those pushing to redefine marriage went as far as taking the government to court to stop the Australian people having a say.
Both legal challenges - one led by independent MP Andrew Wilkie and the other by Australian Marriage Equality - failed.
They had wanted the full bench of the High Court to stop the postal survey, arguing the government should not have bypassed parliament in funding it.
The government found the $122 million by using laws to make an advance payment to the finance minister in circumstances where there is an urgent need for spending and the situation was unforeseen.
Public Interest Advocacy Centre CEO Jonathon Hunyor said the case was an important test of the limits of government power.
"This was a very important case to run because it raised fundamental issues about how governments exercise power in this country and about the role of parliament in our democracy, particularly when it comes to deciding important issues of rights," Mr Hunyor said.
"We are obviously disappointed with the outcome but we now need to focus on ending discrimination against same-sex couples and making marriage equality a reality."
Prime Minister Malcolm Turnbull encouraged every Australian to have their say in the survey.
