Prime Minister Malcolm Turnbull has pleaded with both sides of the same-sex marriage debate to campaign respectfully, describing the use of abusive language as hurtful and deplorable.

Anti-LGBTI posters featuring the slogan "stop the fags" have appeared in some Melbourne laneways. The posters claim "92 per cent of children raised by gay parents are abused, 51 per cent have depression, 72 per cent are obese".

As Australians prepare to vote in a postal survey, Mr Turnbull acknowledged people were likely to say things that are "hurtful, unfair and sometimes cruel". But rather than stifle free speech, Mr Turnbull said Australians should stand up for any friends and loved ones feeling distressed "at this challenging time".

The Prime Minister again said he would vote yes in the survey and declared a bill to legalise same-sex marriage would "sail through Parliament" if most Australians backed the change. He said Australians should not get distracted by "a handful of extreme posters and flyers" and should avoid "caricaturing" those on either side of the debate.

He also had a message for colleagues who have warned the change could have "wider ramifications", saying the biggest threats to traditional marriage were "desertion, cruelty, neglect, abandonment and indifference".

Former prime minister Tony Abbott has suggested both freedom of religion and freedom of speech would be at risk if same-sex marriage is legalised, and told Australians to vote No if they do not like the direction in which the country is going.