PREMIER Jay Weatherill is backing the legality of the latest attempt to legalise same-sex marriage in South Australia, to be introduced in State Parliament today.

Labor backbencher and close Weatherill ally Susan Close has drafted the Same Sex Marriage Bill 2013 which would enable homosexual adults who marry in SA to be registered with the Births, Deaths and Marriages Office and receive a marriage certificate.

Dr Close said she had received legal advice that the SA Parliament was within its rights to pass the Bill.

YOUR SAY: Do you think SA should support gay marriage?

Mr Weatherill, who will vote in favour of the Bill, says loving couples should have the right to have their relationships legally and publicly recognised.

"I accept that there may be contrary legal views. Ultimately, questions of constitutionality are resolved by the High Court in the event that someone decides to challenge a law,'' he said.

Federal law defines marriage as between a man and a woman but constitutional law experts say there is room for a state to create its own laws about other forms of marriage.

If the legislation passes, SA would become the first Australian state to legalise gay marriage.

University of NSW constitutional law expert Professor George Williams said while the legal ground was uncertain, there was room for debate.

"(One) argument is that the federal law deals with marriage between a man and a woman so state law is left to deal with other forms of marriage," he said.

Earlier, Upper House MPs supported, by 11 votes to 10, a motion to commend the New Zealand government legalising gay marriage.

At least 19 Lower House MPs are on the record as supporting gay marriage, including Opposition Leader Steven Marshall.

Dr Close introduces the Bill today on the 35th birthday of her younger brother Stephen, who is gay.

"It is a jarring injustice to me (is) ... that I am able to marry the person I love but Stephen is not," she will tell Parliament this morning.

Labor MPs will get a conscience vote on Dr Close's proposed legislation but Liberal MPs will be required to vote along party lines.

Liberal justice spokesman Stephen Wade said Liberal MPs were "free to express their views, whether it is for or against" but passing state laws legalising same-sex marriage "would be playing a cruel and misleading trick" because they could be overruled by federal laws.

A vote is expected by the end of the year, before the state election next March.

media_camera Port Adelaide MP Susan Close will introduce the legislation.

Port Adelaide MP Susan Close will introduce the legislation. Picture: adelaidenow

The proposed Bill defines same sex-marriage in South Australia as "the lawful union of two persons to the exclusion of all others, voluntarily entered into for life".

Celebrants would have to register as a same-sex marriage celebrant to perform a gay marriage but religious ministers would not be forced to marry gay couples.

Greens MLC Tammy Franks will introduce identical legislation to the Upper House.

Ms Franks said the latest legislation was the state's best chance at reform.

"We can never know whether or not, at the end of the day, it can withstand (a High Court challenge) but we're giving it the best chance possible that we're not crossing any constitutional boundaries," she said. "We believe that there's a lot of people in the South Australian parliament who do support marriage equality in principle.

"We believe it will be closer here than it has been elsewhere."

University of Adelaide constitutional law expert Professor John Williams, who consulted with Dr Close on the Bill, said both the Commonwealth and the states had the power to legislate over marriage. Where a conflict occurs, Commonwealth law usually prevails.

However, Professor Williams said SA could legally pass a gay marriage law and test its validity in the High Court. A decision there would set a national precedent.

"Nobody knows what the answer to that is because there's not been a successfully passed state Act," he said.

Australian law does not allow states to recognise same-sex marriages performed overseas but the Greens have a Bill before the Senate that would overturn that.

DLP Senator John Madigan will also today give notice he will introduce a Bill next week that would seek a referendum on the divisive issue.

Last September the Senate voted down a bill to legalise same sex marriage 41 votes to 26.

####