The Labor frontbencher Terri Butler has hit out at the former Australian 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, Howard claimed there was 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.

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.”

Butler said 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,” Butler said.

“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.”

On Sunday the acting special minister of state, Mathias Cormann, defended Howard, saying he was “entitled to express his view” and the criticism of him was “unwarranted and uncalled for”.

“Everyone is entitled to participate and express their views,” he told Sky News. “We said we expected people to do so with courtesy and respect, and of course [that’s what] John Howard was doing.”

Ahead of the Australian Bureau of Statistics releasing the first update on the participation rate in the marriage law survey on Tuesday, Cormann said that “anecdotally ... a very large number of Australians” had immediately returned their ballots after they were mailed out from 12 September.

Guardian Australia has reported the yes campaign believes a majority of enrolled Australians have already returned their survey form and polls suggest final turnout could be above 80%.

Cormann said there had been a “a high level of enthusiasm” in the postal survey and he expected the final turnout would be a “credible participation rate”.

He said that the government had “clearly indicated” it would facilitate consideration of a private member’s bill to legalise same-sex marriage if the yes vote was higher, regardless of the margin or turnout.

Both the current prime minister, Malcolm Turnbull, and the 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 Guardian Australia, showed 58% of Australians support a change to allow same-sex couples to marry, with 72% of those who had already voted indicating they had voted yes.

Shorten and the Greens leader, Richard Di Natale, will headline a rally in favour of marriage equality on Sunday in Melbourne, with speeches at the State Library of Victoria starting at 1pm.

From there, the crowd will march through the city to Treasury Gardens where a DJ will “spread some positivity”.

“We are expecting to see Australia’s largest ever rainbow down Swanston Street,” Equal Love convenor Ali Hogg said. “We’ve got 100m of material in each colour of the rainbow.

“We know we’re stronger when we’re together, so if people are getting down by anything to do with this postal survey, they can get together and feel the community spirit.”