UPDATE 10.05am: PRIME Minister Julia Gillard is under attack from the Australian Greens and activist groups for pushing for a conscience vote in parliament on same-sex marriage.

Ms Gillard will ask next month's ALP national conference to support a conscience vote, even though she won't back a change to existing laws.

"My position flows from my strong conviction that the institution of marriage has come to have a particular meaning and standing in our culture and nation and that should continue unchanged,'' she wrote in an opinion piece published in Fairfax media.

But Ms Gillard said it was appropriate that a conscience vote flow to Labor parliamentarians.

"They should be free to vote in the parliament according to their own values and beliefs,'' she said.

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Her stance has angered the Australian Greens, who support same-sex marriage.

"It seems that after years of blocking a conscience vote on the issue in the absence of a progressive party platform, the PM is now willing to use a conscience vote to block reform itself,'' Greens senator Sarah Hanson-Young said.

Ms Gillard knew that unless the Coalition granted its members a conscience vote, any vote to amend the Marriage Act would fail.

"It seems she is banking on (Opposition Leader) Tony Abbott continuing to say no,'' Senator Hanson-Young said.

The Australian Marriage Equality says Ms Gillard has pre-empted Labor's national conference by calling for a conscience vote.

"Ms Gillard appears to be captive of the extreme right of the Labor Party led by union boss Joe De Bruyn,'' national convenor Alex Greenwich said in a statement.

That position was at odds with the mainstream of the party and public opinion.

A majority of Australians and Labor members wanted the prime minister to lead the country towards equality, not support outcomes designed to scuttle progress, Mr Greenwich said.

According to a poll published in Fairfax media, 62 per cent of Australian voters support gay marriage, up from 57 per cent a year ago, with 31 per cent opposing.

Among Labor voters 71 per cent support same-sex unions, among Coalition voters 50 per cent are in favour (with 44% opposed) and 86% of Greens support same sex marriage.

Greens leader Bob Brown said Ms Gillard should join the 21st century and advocate for marriage equality.

"It's going to happen, it's just a matter of when,'' he told ABC radio.

Liberal frontbencher George Brandis said Labor was "wobbly on the traditional definition of marriage'' and the issue would become a point of difference for voters.

He said the Coalition's position on the issue was simple.

"We support the traditional definition of marriage,'' he told Sky News.

Trade Minister Craig Emerson said he personally agreed with the Prime Minister's view and welcomed the conscience vote.

"The Liberal Party for years has said Labor locks people in and aren't allowed to have any ideas or individual values,'' he said.

"But when we announce a conscience vote they say it's shocking.''