The time has come for the society to accept as norm the practice of gays entering into legal marriage, says South Australia's outgoing premier, Mike Rann.

In his 2011 Flinders University Investigator Lecture in Adelaide on Sunday, Rann urged the Parliament to update the Marriage Act and allow provision for homosexual couples to marry. Rann's speech concludes the biennial Adelaide Festival of Ideas.

"Same-sex marriage is an idea whose time has well and truly arrived... Not to address this discrimination will diminish us as a nation, as a tolerant community."

Rann insists that there should be no disparity in the legal recognition of the union of a man and a woman and that of homosexuals. Saying those who disagree still has his respect, Rann said civil unions were like a natural resort for those who want to see marriage reserved exclusively for relationships between a man and a woman, reports ABC News.

"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?"

Rann said the State Government had embarked on a series of law reforms after winning office in 2002, including changing over 90 laws to give same-sex couples equality in matters relating to property, superannuation benefits and compensation. But these are not enough without recognition of the rights of same-sex couples, he said.

Rann, who will step down as premier on October 20 has just celebrated his 17 years as SA Labor leader.

The issue on gay marriage will be debated at Labor's national conference in December.

The Victorian Labor branch on Saturday 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, according to ABC.