South Australia's outgoing premier has called for the Federal Government to legalise same-sex marriage.

Delivering the 2011 Flinders University Investigator Lecture in Adelaide on Sunday, Mike Rann called on the federal Parliament to change the Marriage Act to allow homosexual couples to marry.

"Same-sex marriage is an idea whose time has well and truly arrived," he said during the speech, which marks the end of the Adelaide Festival of Ideas.

"Not to address this discrimination will diminish us as a nation, as a tolerant community."

Mr Rann said that while he respected those who disagreed, he thought civil unions were an escape clause for those who want to see marriage reserved exclusively for relationships between a man and a woman.

"It is, quite simply, unfair to prevent same-sex couples from having their relationship, a union that is viewed as equal in every other aspect of the law, being recognised as a legal marriage," he said.

"It only serves to undermine the legitimacy of their relationship, and their family. What are we so afraid of? Why is this next step so threatening?"

Mr Rann said the State Government had embarked on a series of law reforms after winning office in 2002.

These included changing more than 90 laws to give same-sex couples equality in matters relating to property, superannuation benefits and compensation.

However, Mr Rann said he was not satisfied that these reforms went nearly far enough to recognise the rights of same-sex couples.

Federal Labor frontbencher Craig Emerson said Mr Rann was entitled to his view on marriage, but neither he nor the Prime Minister agreed with the SA Premier.

"These are the sorts of issues that are debated in the community reflecting a great diversity of view within the community and that diversity of view within the community is reflected in the Labor Party and I see nothing wrong with having a debate about these issues, but my position won't change and that is that marriage is between a man and a woman," he said.

A former ALP deputy leader in South Australia, Ralph Clarke, said Mr Rann's endorsement of same-sex marriage came far too late.

"He's 100 per cent right and I welcome it, however I think that that sort of support should've been given years ago," he said.

"It's not something that you make up at the end of your political career."

One of Mr Rann's current Labor colleagues Stephanie Key says same-sex couples in South Australia still cannot adopt children, unlike in other states such as New South Wales.

"Many of us in the ALP believe that it's important gay couples have the same parenting rights, including adoption rights, as anyone else," she said.

The gay marriage issue will be debated at Labor's national conference in December, with SA's first openly-gay Labor MP Ian Hunter expected to attend.

The Victorian Labor branch yesterday voted to overhaul the party's policy on gay marriage.

Queensland Premier Anna Bligh, Tasmanian Premier Lara Giddings and ACT Chief Minister Katy Gallagher have also expressed their support for gay marriage.

Mr Rann, who will step down as premier on October 20, last month celebrated 17 years as SA Labor leader.

AAP/ABC