Former prime minister Tony Abbott's sister Christine Forster has accused same-sex marriage opponents of fearmongering, as a string of high-profile Yes campaigners staged rallies across the country to urge supporters to remember to return their survey forms.

Key points: Christine Forster, a Liberal councillor, accuses the No campaign of trying to confuse voters

Christine Forster, a Liberal councillor, accuses the No campaign of trying to confuse voters Rallies around the country held by Yes campaign to remind people to post their votes

Rallies around the country held by Yes campaign to remind people to post their votes Comes a day after official No campaign launch in Sydney

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."

Key dates in SSM postal survey: August 24 — the final day to register with the AEC if you want to take part in the survey

September 12 — survey forms start being sent out

September 25 — all forms are expected to have been sent

October 27 — forms are strongly encouraged to be returned by this date

November 7 — the final deadline to return surveys

November 15 — results are released

Her comments came a day after Senator Corey Bernardi told an audience of 1,000 people, gathered for the No campaign launch in Sydney, they would face "weaponised" anti-discrimination laws and "legal warfare" if the marriage act was changed.

Ms Forster later took to Twitter to condemn her brother, who is advocating for 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 believe in the institution of marriage, that's why I'm here. I was raised in a family, my parents were married for 58 years, and I would hope if I meet the woman of my dreams that I could have that same institution there to back me up and support me," she told the ABC.

"And I would hope that you would want that for me too."

In Adelaide, a trio of federal MPs — the Liberals' 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 [a survey form] 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.

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AAP