Prime Minister Malcolm Turnbull will introduce legislation to enable a plebiscite on same-sex marriage this week, despite so far failing to secure crucial support from the Opposition.

Labor is strongly hinting its intention to block the vote, but will wait until the detail of the legislation is revealed.

Without Labor's support the legislation is not expected to clear the Upper House.

Opposition Leader Bill Shorten has said he has "grave concerns" about the plebiscite and it could be some time before the final Labor position is known.

Once the bill has been put forward, Mr Shorten intends to take it to his caucus room. He said that was likely to happen at the next regular meeting in three weeks.

The Prime Minister and Attorney-General George Brandis have both appealed to Labor to back the plebiscite bill.

"The only person that stands in the way of Australians having a vote on the issue, on February 11, is Bill Shorten," Mr Turnbull said.

Senator Brandis urged Mr Shorten to "get out of the way" and while he conceded the vote would not be binding, he expressed confidence a "yes" vote would be respected by the Parliament.

"There is no doubt, no doubt whatsoever that if the Australian people choose to vote yes, the Parliament — both sides of Parliament — will respect their wishes," he said.

Liberal Senator prepared to cross the floor

Sorry, this video has expired Brandis says plebiscite offers 'completely level playing field'

Senator Brandis said the Government was "united" on the process, despite Liberal senator Dean Smith saying he would not vote for the legislation.

Senator Smith's office has confirmed he would either abstain or cross the floor on the "abhorrent" issue, something he forecast earlier this week.

The Greens, crossbench Senator Derryn Hinch and the trio of Nick Xenophon Team senators have also pledged to vote against the bill.

Sorry, this video has expired Adam Bandt confronts PM on plebiscite

It means that without the support of Labor, the Coalition will not have the additional nine votes it needs to pass legislation in the Upper House.

If the Government manages to secure parliamentary support for the bill, the compulsory national vote will be held on February 11 and will be determined by a simple majority of votes cast.

There will be $15 million in funding for the yes and no campaigns — $7.5 million each — and the question to be put to voters will be: "Should the law be changed to allow same-sex couples to marry?"

In the House of Representatives, Labor used Question Time to press the Government on the plebiscite, with Deputy Labor leader Tanya Plibersek speaking about a gay couple and their son, who were in the public gallery.