Opponents of same-sex marriage have officially launched the No campaign, saying they are on the "right side of legal and moral history".

Key points: New TV ad was played at the event and has been posted online

New TV ad was played at the event and has been posted online The new ad talks about "radical sex and gender programs" in schools

The new ad talks about "radical sex and gender programs" in schools Senator Cory Bernardi's speech focused on freedom of speech

Last night Australian Conservatives senator Cory Bernardi asked No supporters to be "vigilant" at an event launching the Coalition for Marriage's campaign at the Sydney Convention Centre.

A new TV ad was played at the event and has been posted online, and on Sunday morning Sydneysiders could not miss "Vote No" sprawled across the sky.

"We are under assault because we are on the right side of legal and moral history," Senator Bernardi told an audience of more than 1,000 people.

Australian Conservatives Party leader Cory Bernardi spoke at the No campaign launch. ( AAP: Perry Duffin )

"It is a sign of what is to come unless we are vigilant and can defeat this vote in the weeks ahead."

The campaign launched after many Australians started to receive their same-sex marriage postal surveys in the mail.

Senator Bernardi's speech focused on freedom of speech which he said would be affected by a change to the Marriage Act.

"Many of you know that I am a champion of freedom," Senator Bernardi said.

"Whilst I may struggle to love my enemies I always defend their right to freedom of speech.

"If the state redefines marriage it also redefines how you can speak, think, advocate, and believe about marriage."

'Gay and lesbian friends already have equality'

Lyle Shelton, head of the Australian Christian Lobby and spokesman for the Coalition for Marriage, also drew links between the marriage laws and changes to discrimination laws.

"Gay and lesbian friends already have freedom and equality under the law in Australia but if we change the definition of marriage the freedom and the rights of other Australians will change forever," Mr Shelton said.

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

Liberal Party senator Concetta Fierravanti-Wells also spoke in support of the No campaign.

Senator Fierravanti-Wells said she wanted to protect the "cornerstone" family values of the migrant populations.

"I believe that marriage is between a man and a woman … coming together in one unique union," she said to the crowd.

"That is what it has been for every culture, in every ethnicity, in every faith in every corner of the world for thousands and thousands of years."

"It's OK to vote no," Senator Fierravanti-Wells said, reflecting the No campaign slogan.

No advertisement targets 'radical programs'

The Coalition of Marriage's new ad talks about "radical sex and gender programs" in schools and does not mention the issue of marriage.

Loading...

"How am I supposed to protect my kids in the future from this stuff?" one of the mothers in the ad asks.

The ad posting on the Coalition for Marriage Facebook page had mixed comments from both sides of the debate.

Shortly before midday on Sunday a skywriter wrote "Vote No" multiple times across Sydney.

Mr Shelton said the Coalition for Marriage did not organise the skywriter and he was not sure who had paid for it.

The sky message attracted a lot of attention on social media both for and against the No campaign.

Loading

Loading

Loading

Loading