Marriage equality advocates said they would continue to argue for change but refused to say whether they would encourage supporters to vote in the plebiscite.

Fairfax Media last year revealed former Liberal MP Chris Miles had authorised pamphlets that seek to sow doubts about the social consequences of legalising same-sex marriage.

The pamphlet reads: "Married biological parents have a better record for providing safety and development of healthy, well-adjusted adult children. They minimise abuse and neglect of children."

It also suggests the children of gay parents are more likely to use drugs, be unemployed and suffer depression.

Mr Miles on Tuesday said planning was "well under way" for a campaign and that he expected to print many more copies of the pamphlet to distribute during a campaign.

"The work has been going on quietly, but now there is a go-ahead we will think about how to proceed over the next two to three months," he said. "There is a lot more enthusiasm now."

Opposition Leader Bill Shorten blasted the pamphlet as "disgusting ... vile rubbish" and said it showed there would not be a respectful debate during a plebiscite campaign.

Prime Minister Malcolm Turnbull said: "We respect Australians' ability to have a frank and forthright discussion on this issue.

"We're not going to shut down democracy and debate because people here or there say outrageous things or defamatory things."

Australian Christian Lobby executive director Lyle Shelton said his organisation was planning to use the "full gamut" of campaigning techniques from traditional media advertising to pamphlets to online messaging.

He said it would be "terrible" if media outlets refused to run anti-same-sex marriage advertisements, as public broadcaster SBS did during the 2015 Mardi Gras parade.

"Both sides should be given an equal opportunity to get the message out," he said.

Mr Shelton said the ACL campaign would focus on the consequences of changing the definition of marriage. He said these would include censoring teaching in Christian and Muslim schools, making it easier for people to change their sex and the proliferation of Safe Schools-style programs.

"All of this flies under the rainbow flag," he said, adding the campaign would be "respectful to our fellow Australians".

"We will be asking mums and dads to donate, it will be very grassroots," he said.

"We don't have the support of corporate Australia like other [pro-same sex marriage] groups. This is a David v Goliath battle."

The government's original proposal for a compulsory plebiscite at the ballot box included $7.5 million in public funding for the "yes" and "no" campaigns. A postal plebiscite campaign would have no public funding.

Broadcasting legislation would also have been changed to force TV broadcasters to give both sides a "reasonable opportunity" to make their case. There would be no special broadcasting rules in a postal plebiscite campaign.

Asked if his group would boycott a postal plebiscite campaign, Australian Marriage Equality chairman Alex Greenwich said he was "not ruling anything in or out" as he waited for more detail.

"How can we have confidence in this process when the Prime Minister himself has said he's too busy to participate actively in a campaign?" he said.

"How can he expect anyone else to engage in it if he himself is saying he's not willing to?"

Progressive activist group GetUp! - which also opposes a plebiscite - would be expected to deploy its resources to encourage young Australians to vote "Yes".

Same-sex marriage advocates are worried that young voters are more likely to not be enrolled, have changed their address since the last election or not check the post.