In 2008, he told the Express gay newspaper that he saw no need to go further than civil unions.
And at the Big Gay Out festival in Pt Chevalier in February, he told GayNZ.com that civil unions were a "big win", but it was unlikely there would be movement on gay marriage, although "the community" might argue there was no harm in it.
At a Family First forum just before last year's election, he refused to say he would support a bill to define marriage as between men and women.
Mr Key voted against the Civil Unions Bill in 2004, but has said it was because of demand from his electorate rather than his own views.
Labour Party leader David Shearer said he fully supported marriage equality in principle but would like to see the detail of any legislation before giving it formal support.
Labour's justice spokesman, Charles Chauvel, was more direct in his support for same-sex marriage.
"It was Labour policy in the last election to support the right of same-sex couples to marry."
Labour was looking at a private member's bill this term to make it legal for same-sex couples to marry.
"I think the great thing about the President's announcement is that it helps to highlight the issue of equality and keep it on the agenda and more and more New Zealanders are saying it's a no-brainer, people should have these rights," Mr Chauvel said.
Maori Party co-leader Tariana Turia said she would support same-sex marriage, as individuals and whanau had the right to choose for themselves whether to marry.
Greens co-leader Metiria Turei said she was pleased President Obama had taken a stand on what was a very difficult moral issue in America.
"The US is very morally conservative on issues like sexual reproductive health, gay and lesbian rights and it is about time that the President took a strong stand in favour of the community and their right to be treated equally."
She said the Green Party supported same-sex marriage in New Zealand and had argued for it when the Civil Union Bill was being passed.
"Our policy is that same legal rights and responsibilities should apply to all couples regardless of whether that couple is gay, lesbian, transgender or heterosexual," she said.
United Future leader Peter Dunne declined to comment, Act leader John Banks did not respond to questions, and Mana Party leader Hone Harawira said he was undecided.
A poll in the Herald in 2004 found 40 per cent of New Zealanders supported same-sex marriages and 54 per cent were against. A Research New Zealand poll last year found 60 per cent were in favour and 34 per cent against.
