Australians who value the “traditional definition” of marriage and freedom of speech should vote against same-sex marriage, Mr Abbott said, speaking to reporters outside Parliament House on Wednesday morning.
“If you don't like political correctness, vote no,” Mr Abbott said.
The former PM is a long-time opponent of same-sex marriage, but said he would “respect the result” of the public vote.
The Australian Christian Lobby is also gearing up for the “no” campaign, saying it will argue same-sex marriage will lead to changes to the education system and an expansion of the controversial Safe Schools program.
“We will be asking mums and dads to donate, it will be very grassroots,” ACL’s executive director Lyle Shelton told the Sydney Morning Herald.
Prime Minister Malcolm Turnbull will publically support the “yes” vote, but said he had “many other” calls on his time that would limit his capacity to campaign.
Mr Turnbull spoke at an inter-faith breakfast in Canberra on Wednesday with opposition leader Bill Shorten.
“That is when we are closest to God, when we love. When we open our heart and think not of ourselves but of others,” Mr Turnbull said, not making any specific reference to the same-sex marriage debate.
He urged practicing “altruism … to the stranger, to the person we don't know, perhaps don't understand”.
“When we argue with each other, we are arguing and debating about what is the right way forward, what is the best way we can show our love,” Mr Turnbull said.
It comes after the Turnbull government has been forced to adopt its Plan B on same-sex marriage - a postal vote - after the Senate rejected a second bid for a compulsory plebiscite.
Labor, the Greens and crossbenchers blocked the government's motion to bring on the plebiscite laws for a fresh debate on Wednesday, with the vote negatived at 31-all.
Ballot papers are expected to be posted from September 12 with voters given until November 7 to return their ballots and a result declared on November 15.
If there is a majority "yes" vote in the $122 million poll, the parliament would debate and vote on a private member's bill to change marriage laws in the final sittings for the year.