GAY marriage is set to divide both Labor and Liberal, with Premier Jay Weatherill's declaration he will vote for the unions.

It's set to offend Labor's powerful religious conservatives and feed growing Liberal leadership tensions.

Flanked by a pink Australian flag and a banner promoting socialist agitators Resistance, Mr Weatherill stood on Parliament House steps yesterday to declare he will vote in favour of a Bill to legalise gay marriage now before State Parliament.

His pledge to vote for the Greens' Bill now in the Upper House adds weight to the campaign - Labor will allow MPs a conscience vote in a party where the conservative Catholic Right wing dominates and key members are in seats with significant blocs of voters opposed to the idea.

His declaration coincided with his Tasmanian counterpart also assuring supporters to legislate for same-sex marriage regardless of what federal parliament does.

Mr Weatherill, from the minority Left faction, has previously publicly supported the principle of gay marriage but his endorsement as Premier of the Greens' Bill gives the debate new strength. He said he expected "a number" of fellow Labor MPs would support the Bill.

"I support the principle of marriage equality," he told the cheering crowd at the Equal Love rally, while warning that any society that did not allow individuals to develop emotionally consigned them to being second-class citizens.

"People should be entitled to express their own identity in any way they wish and the law should not be a barrier to prevent them from doing that." Mr Weatherill confirmed SA had not sought advice on whether the state could legally enshrine gay marriage given that the Commonwealth laws states marriage must be between a man and a woman.

"We don't have any legal advice," he said.

While Prime Minister Julia Gillard opposes same-sex marriage, Mr Weatherill said it was supported by the party platform, subject to a conscience vote on any legislation by MPs. Opposition Leader Isobel Redmond remained out of contact yesterday - she has flown interstate and is not speaking to the media - amid growing speculation about her grip on the job and concern from some MPs about whether they are presenting a distinct alternative government.

Former leader Martin Hamilton-Smith stepped into this vacuum to attack Mr Weatherill over his public support for gay marriage.

Stressing his respect for gay couples, Mr Hamilton-Smith said many people would be disappointed to see Mr Weatherill moving from an individual view to public advocacy by using his position to promote the cause.

"The leader of the Labor movement generally speaks on behalf of all ALP members - what message do other Labor MPs' have for their constituents?" Mr Hamilton-Smith said.

"While respecting the Premier's view I disagree with him completely. I strongly support our current legal definition of marriage."

Originally published as Gay marriage 'divisive' for parties