Tasmania's Lower House looks likely to become the first in Australia to vote in support of same-sex marriage.

Labor has indicated it will support a motion from its state power-sharing partners the Greens, who have tried and failed to introduce state-based same-sex marriage legislation in the past.

Greens leader Nick McKim last week announced the party would bring on a motion in Parliament this week expressing in-principle support for marriage equality and calling on the Federal Government to amend the Marriage Act.

If he succeeds, Tasmania's Lower House will become the first state Lower House in Australia to vote in support of same-sex marriage.

The motion is expected to be debated by Parliament tomorrow, but is unlikely to have Liberal support and there will be no Liberal conscience vote.

But the Greens' cause was helped when same-sex marriage was recently backed at the Tasmanian Labor Party's state conference.

Premier Lara Giddings says this is the position the Government will take to the Greens' motion.

In a statement, Ms Giddings said Tasmanian Labor supported the principle of same-sex marriage on the basis that it provided equality.

Like every other Labor branch in the country except for New South Wales, the Tasmanian ALP has supported same-sex marriage at its state conference.

Labor's support means the motion will be passed by the Parliament tomorrow, but the support will not be unanimous.

Sorry, this audio has expired Tasmanian parliament set to support same sex marriage ( Felicity Ogilvie )

Deputy Opposition Leader Jeremy Rockliff says the Liberals believe marriage is between a man and a woman.

He describes Mr McKim's motion as a backdown from the Greens leader's attempts to legalise same-sex marriage in Tasmania.

"I find it extraordinary, firstly, that Mr McKim has backed down with respect to his legislation that he's tabled and not willing to debate that legislation," Mr Rockliff said.

"It seems to be a backdown now for a motion to be debated in the house tomorrow ... which would appear to be a compromise between Labor and the Greens."

But Mr McKim says his motion is not a compromise.

"What we are doing is seeking to be part of a national campaign here in order that the best possible outcome can occur, and that best possible outcome is for this issue to be resolved at a national level," he said.

"If that doesn't happen by the end of this year, then we will move ahead with our state-based same-sex marriage legislation in Tasmania and give the Parliament of Tasmania the opportunity to take a national lead."

Ms Giddings has said it was clear from constitutional advice that any lasting changes to the Marriage Act would have to occur at a national level.