Labor frontbencher Terri Butler has hit out at former prime minister John Howard for taking out ads in major newspapers calling for people to vote 'no' in the postal survey on same sex marriage.

In the ads, Mr Howard claims there is less "generosity of spirit" on the 'yes' side of the debate, because Labor and the Greens have refused to accept a 'no' vote and would move to introduce same sex marriage even if the majority of people vote 'no'.

Mr Howard argues the current parliament must ensure religious freedoms are protected if the 'yes' vote wins.

"The issue must be addressed before the survey is completed," he said in the ad.

"Leaving it as something to be taken up only in the event of a 'yes' vote prevailing is the equivalent of saying it does not matter very much."

Ms Butler said Mr Howard was "yesterday's man" and while entitled to his opinion, it was "unfortunate" he had used his standing as a former prime minister to advocate for the 'no' campaign.

"It was, as you know, the Howard government which changed the marriage act a long time ago to explicitly say the marriage is between a man and a woman," Ms Butler told reporters in Brisbane on Saturday

"It is a bit rich coming from someone who did change the marriage law without any need for a national opinion poll or a plebiscite or anything like that."

Both current Prime Minister Malcolm Turnbull and Opposition Leader Bill Shorten have promised to preserve religious freedoms if same sex marriage is introduced.

The most recent poll on the issue, conducted by Essential for the Guardian, showed 58 per cent of Australians support a change to allow same sex couples to marry, with 72 per cent of those who had already voted indicating they had voted 'yes'.