More than a dozen members of the Coalition are preparing to support a same-sex marriage bill, but not the one the Prime Minister hopes to pass through Parliament.

Key points: Government expected to support Liberal senator Dean Smith's bill to legalise same-sex marriage if it goes to a vote

Government expected to support Liberal senator Dean Smith's bill to legalise same-sex marriage if it goes to a vote Conservative Liberal Ian Goodenough says an alternative bill includes "expansive protections" for religious freedoms

Conservative Liberal Ian Goodenough says an alternative bill includes "expansive protections" for religious freedoms Warren Entsch, who helped draft Senator Smith's bill, says the "alternative" has come too late

However, a long-time supporter of same-sex marriage has lashed out at his colleagues' plan, arguing it would have more "credibility" if it was not "rushed together".

The ABC understands the group is putting the finishing touches on the bill, which proponents say includes "expansive protections" for so-called religious freedoms.

West Australian Liberal MP Ian Goodenough told AM he would support the bill, which would be presented to Parliament if next week's same-sex marriage survey returns a Yes vote.

He said more than a dozen members "at this stage" would join him, but he would not say exactly who would sponsor the bill.

WA Liberal MP Ian Goodenough is one of "more than a dozen" conservatives who could back the bill. ( ABC News: Matt Roberts )

Conservatives are not happy with WA Liberal senator Dean Smith's bill, which the Government is expected to use to legalise same-sex marriage if the issue goes to a vote.

"I believe the bill is focused on the wedding ceremony itself but it doesn't go further than that," Mr Goodenough said.

But Government backbencher Warren Entsch, who helped draft Senator Smith's bill, said the alternative plan has come too late.

"Just because you've got a small group of individuals there that suddenly they've decided they want to actually put a bill up on same-sex marriage, who have stridently opposed it forever, and now expecting the Australian public to believe that they are suddenly committing to making it happen, that is not the broad view of the Coalition," Mr Entsch said.

Senator Smith's bill allows same-sex couples to wed, while creating exemptions so that religious organisations can refuse to conduct same-sex marriages.

Labor has agreed to back the bill if it comes up for debate and frontbencher Penny Wong said the Opposition would not consider another "delaying and blocking tactic from the same people who have opposed equality".

"Let's be very clear, if the survey is a Yes, Australians are not going to tolerate this group of MPs and senators blocking progress again," Senator Wong said.

"If the No case loses the survey, they need to get over it and they need to get out of the way."

Conservatives are worried the legislation could see same-sex marriage opponents fall foul of the law if they air their views in public.

Faith-based charities' funding 'would be under threat'

The ABC has obtained a list of "talking points" criticising Senator Smith's bill, which have been circulated amongst opponents of the proposal.

Mr Goodenough said the alternative bill contains protections for parents and schools with strong religious views.

"To allow those schools to teach their students about the traditional view of marriage without necessarily having to provide the full spectrum of views on marriage."

Leaked talking points criticising the Smith bill It does not protect the religious freedom of most Australians

It does not protect the religious freedom of most Australians Civil marriage celebrants who do not want to solemnise same-sex marriages for religious reasons must make this objection public

Civil marriage celebrants who do not want to solemnise same-sex marriages for religious reasons must make this objection public It provides insufficient protection for religious bodies

It provides insufficient protection for religious bodies The bill fails to address non-religious objections to same-sex marriage

The bill fails to address non-religious objections to same-sex marriage The limited safeguards provided to clergy and religious celebrants do not override state anti-discrimination laws

The limited safeguards provided to clergy and religious celebrants do not override state anti-discrimination laws The bill ignores the Senate Select Committee report's call for additional religious freedom protections

The bill ignores the Senate Select Committee report's call for additional religious freedom protections It fails to address the interaction with state-based anti-discrimination laws

Mr Goodenough said the bill also includes amendments to protect faith-based charities like the Salvation Army, Anglicare and St Vincent de Paul from losing funding.

"Any government funding for these programs would be under threat," Mr Goodenough said.

"I think a number of [overseas] charities may have lost their charitable status due to inconsistencies with marriage legislation, so they were faced with the dilemma of having to change their constitution, if you like, in order to comply with the legislation."

Mr Entsch said Australians wanted the issue dealt with if the postal survey returned a Yes vote.

"There will be very little tolerance within the Australian public if the majority of people are saying yes, then you're going to find a group of individuals that are going to continue to try and manipulate the process or invent further ways of kicking the can down the road," Mr Entsch said.

If the alternative same-sex marriage bill is presented to Parliament, it could further fuel the ongoing feud between moderates and conservatives in the Coalition.

It could also delay Prime Minister Malcolm Turnbull's plan to allow a free vote in Parliament before Christmas.

Mr Goodenough said he did not want the process rushed and said it was "possible" the alternative bill could be presented to the Senate next week.

"It'll take several weeks to get even a dozen amendments through if they're contentious," he said.

"So suddenly it'll be very difficult and challenging to get the required number of detailed amendments through before Parliament rises."

But the ABC understands senior Government figures have been expecting the challenge, and they are prepared to head it off.