A legal challenge will be launched tomorrow if today's Liberal Party meeting on same-sex marriage decides to have a national postal ballot.

Key points: Liberal MPs have returned to Canberra early for a special meeting on same-sex marriage

Liberal MPs have returned to Canberra early for a special meeting on same-sex marriage Marriage equality groups vow to challenge any postal ballot in court

Marriage equality groups vow to challenge any postal ballot in court MPs are concerned voters will punish the party if it abandons the plebiscite

The Australian Marriage Equality group, which campaigns for changing the law to allow same-sex marriage, has announced it will launch an immediate challenge to any move to put the issue to a postal ballot.

Liberals are meeting this afternoon to try to resolve the stand-off within the party about marriage.

Most Liberals are likely to argue that the Coalition should maintain its promise to hold a plebiscite where all voters can have a say.

But with the Senate likely to block that plan again, the fall-back option is for a postal ballot.

Alex Greenwhich, co-chair of Australian Marriage Equality, called it "a bloody stupid idea that will weaken the Parliament because it basically says people are not prepared to do their job".

Anna Brown, the director of legal advocacy at the Human Rights Law Centre, has released legal advice that the Government does not have the power to spend money on a postal ballot.

She warned the Government would be exposing itself to a costly legal challenge in the High Court if it went ahead with a postal plebiscite.

Some MPs have said it would be easier to have a postal plebiscite because the Government could ask the electoral commission to organise it without Senate approval.

But Ms Brown argued the Government would still need Senate authorisation to spend the money to arrange the postal ballot.

Same-sex marriage advocates accused of 'kamikaze tactics'

Debate is raging as MPs prepare for this afternoon's meeting and MPs pushing for same-sex marriage have been accused of using kamikaze tactics.

Liberal senator Dean Smith prepared the private member's bill that would allow same-sex couples to marry.

Senator Smith and four other Liberals were arguing for MPs to be given a conscience vote in Parliament on the bill.

They had not ruled out a tactical manoeuvre that would force a parliamentary vote on the bill.

Senator Smith and his colleagues said that was the best way to deal with the issue rather than having a plebiscite or a postal ballot.

Another Liberal, John Alexander, today also called for a conscience vote in Parliament.

"I would prefer a free vote in the Parliament and have it done quickly, then everybody is responsible for the way they've voted, and in the future they'll have to face the music," Mr Alexander said.

Ian Goodenough has raised concerns over the "impossibility of having children without involving a third person". ( ABC News: Ross Nerdal )

But Mr Goodenough told the ABC he was worried about Senator Smith's bill because it did not address the issue of children, "more specifically the impossibility of having children without involving a third person, and related issues such as commercial surrogacy, reproductive technologies and the prospect of taxpayer funding of these services".

"As legislators we must consider what happens after the wedding ceremony and the honeymoon is over — what are the public policy implications for our health system, and every other area [in which] the Government provides services?" he said.

Veteran Liberal MP Warren Entsch has long argued for Parliament to allow same-sex marriage.

But Mr Entsch leaves for a stint at the United Nations in September which has added to the pressure to debate the issue within weeks.

Mr Goodenough objected to that saying the bill was "being rushed to meet someone's travel timetable without the detail being properly considered".

Liberal MPs have returned to Canberra early for a special meeting on same-sex marriage.

Sorry, this video has expired Liberal MP Tim Wilson has also spoken out, saying he wanted to honour his election commitment on same-sex marriage.

Concern over voter backlash

Liberal Craig Kelly warned voters would punish the party if it dumped the policy of having a plebiscite on same-sex marriage.

"We will pay an electoral penalty for that," Mr Kelly told RN.

"Every time you break a commitment to the Australian people there's a transactional cost. Now to go and to break that policy would be a breach of trust to the Australian people."

MPs at this afternoon's meeting are likely to stick to the plebiscite policy.

The Senate refused to agree to a plebiscite last November and is expected to maintain its stance against that position.

The Coalition may decide to have a postal ballot of voters if it cannot get Senate approval for a national plebiscite.

But campaigners for same-sex marriage have already said that might not be constitutional and they would consider challenging it in the High Court.

Former prime minister Tony Abbott also expressed reservations about a postal ballot.

"It's certainly better than ramming it through the Parliament without a vote, but there are questions about how authoritative it would be," Mr Abbott told 2GB.

"We've put the plebiscite up once, that's all.

"We should have another go. If that fails, let's have another go."