Former prime minister Tony Abbott's sister Christine Forster has accused same-sex marriage opponents of scaremongering as a string of high-profile Yes campaigners staged rallies across the country to urge supporters to remember to return their survey forms.
Sports stars, TV identities and politicians of all stripes took part in coordinated events across capital cities as part of the #PostYourYes campaign on Sunday.
Speaking at the Sydney launch, Ms Forster, a Liberal councillor, accused the No campaign of trying to confuse voters.
"The No campaign is trying to muddy the waters with issues that are not related," she said.
"This is a simple question of whether or not you believe the law in Australia should be changed to allow same-sex couples to marry. That is as far as it goes."
Her comments came a day after Senator Corey Bernardi told an audience of 1000 people gathered for the No campaign launch in Sydney they would face "weaponised" anti-discrimination laws and "legal warfare" is the marriage act was changed.
Ms Forster later took to Twitter to condemn her brother, who is advocating a No vote, for claiming protections for religious freedom should have been put in place before the survey was launched.
"A plebiscite was your idea @TonyAbbottMHR As a legislator if you thought protections were needed why haven't you put them in place? " she wrote.
At the Yes campaign's Melbourne event, Magda Szubanski, joining tennis legend Frank Sedgman and Hawthorn great Russell Green, said she hoped Australians would support the change.
"I just hope people can find it in their hearts to understand where we're coming from, which is about ... for the people who want marriage equality it's about strengthening marriage," she told AAP.
In Adelaide, a trio of federal MPs - the Liberal's Christopher Pyne, Labor's Penny Wong and the Greens' Sarah Hanson-Young - put aside political differences to back a Yes vote.
"There's no point leaving it lying on the kitchen bench or stuck on the fridge," Senator Hanson-Young told reporters.
"It won't make a difference there. But it will make a difference if you put it back into that post box."
In Brisbane, Olympic swimmers Emily Seebohm and Mitch Larkin said a close friend had asked them to throw their support behind the campaign.
"We're big fans of equality in sport, and I think it's time to show that equality in the marital scene," Larkin said.
On Saturday night, Senator Bernardi joined prominent coalition politicians Matt Canavan and Concetta Fierravanti-Wells in addressing the no campaign launch.
Senator Bernardi painted the Yes campaign as an attack on free speech but said same-sex marriage opponents held the moral high-ground.
"We're under assault because we're on the right side of legal and moral history," he said.