28/08/2016 7:08 AM AEST | Updated 28/08/2016 3:26 PM AEST

Turnbull Denies Marriage Equality Plebiscite Is 'Doomed'

The ALP is set to join the Greens against the bill.

Stringer . / Reuters
The PM is backing his chances of getting a national vote on same-sex marriage.

Prime Minister Malcolm Turnbull remains confident he can pass legislation for a national vote on marriage equality, despite reports Labor will join the Greens in blocking the proposal.

Media reports on Sunday indicate that plans for a national vote on same-sex marriage look to be facing an uphill battle, with opposition leader Bill Shorten indicating Labor will oppose the move.

Shorten has told Fairfax Media that Labor is considering opposing the plebiscite plan because of fears a popular vote could lose and then stymie momentum towards gay marriage entering law.

But speaking on Sunday, Turnbull stayed upbeat about the plebiscite's chances, denying it was "doomed".

"The legislation will be introduced. It will be passed in the House and then we'll have to see how it goes in the Senate. I think Labor will support it," Turnbull told ABC television.

"I have no doubt the plebiscite will be carried and the same-sex marriage legislation will then sale through the parliament."

The comments come after Shorten told The Sydney Morning Herald his opposition to a national vote had increased and that it's likely Labor would announce it was not backing the government's legislation.

"I'm worried Malcolm Turnbull will just stuff it up," Mr Shorten is quoted as saying. "He stuffed up the republic referendum, he stuffed up the NBN and he stuffed up Senate reforms when he promised to fix it."

Shorten is said to be especially worried that the coalition, which contains many MPs who don't want marriage equality to become a reality, won't vigorously support the campaign.

If the vote fails, Labor is reportedly concerned it would make the cause harder to legislate down the track.

Shorten told reporters in Melbourne that a plebiscite was "the second best option" to get a resolution on marriage equality.

He queried why $250 million should be spent on a national vote and $52 fines imposed on those who didn't go to the polls on the issue when it could be decided by MPs.

"This country doesn't have the time or the money to spend $250 million," he said.

Labor MP Ed Husic on Sunday reiterated his party's support for a parliamentary vote on the issue.

"I don't think people want to see a situation where we have potentially a lot of hate ... directed towards people preferring a same sex or marriage equality outcome," he told the ABC.

"We don't think it should provide for that type of divisive speech and we just think that the best way to avoid all this is for the Parliament to do its job, vote on amending the Marriage Act, provide for marriage equality and don't make us go through a divisive plebiscite."

The report follows calls earlier this week for Labor and the crossbench to join the Greens in voting down the government's proposed plebiscite.

On Friday, Greens Leader Richard Di Natale announced his party would not support a motion to enable the plebiscite.

"No matter what the enabling legislation for a plebiscite looks like, the Greens will vote against it. We should never put questions of human rights to an opinion poll," Di Natale said in a statement.

"The easiest, simplest, quickest, most effective, least costly and least harmful way of ensuring equality in marriage is through a vote in the parliament, and we can do that next week should the Prime Minister decide to show some leadership."

Marriage equality advocate Rodney Croome from just.equal rejects the Government's claim that the plebiscite is the quickest or only way forward for legalising same-sex marriage.

"If the Prime Minister is really concerned about achieving marriage equality as quickly as possible he will have a back-up plan should a plebiscite be vetoed, and that 'Plan B' should be to allow a free vote in parliament."

"The risk there isn't a free vote is a risk the LGBTI community is willing to take to avoid the hurt, harm and indignity of a plebiscite," he said.

Turnbull has previously said he expects marriage equality to "sail through" the parliament if a majority of Australians voted in favour of the plebiscite.

"There are few things in politics that are certain, but one thing that I would say is an absolute certainty is that if the plebiscite is carried by the Australian people, same-sex marriage will be legislated for by the Australian Parliament," he said a few days before the July 2 election.