NATIONAL union boss Paul Howes will compare same-sex marriage to Australia's long-gone White Australia Policy as he calls for the ALP to bind themselves to support equality.
The head of the Australian Workers' Union published the speech he was expected to make at the Future of Marriage Equality Forum in Sydney.
He called for the ALP to dump its planned conscience vote on same-sex marriage in favour of tying the party to its support.
"Conscience votes should be reserved for those very rare issues of individual moral judgement, in which the issue at hand simply defies objective boundaries," his speech read.
"(Marriage equality) is not an issue of individual conscience; it is a matter of basic social justice."
But while Mr Howes' views garnered support from the Greens, aspiring Opposition Leader Bill Shorten and Labor colleagues were unconvinced.
Mr Shorten's office told national media a binding vote "would not be consistent with his vision for a diverse, inclusive Labor Party" although Mr Shorten himself supported the reform.
Frontbencher Tony Burke told ABC Radio it would "undermine community support for the party".
In his speech, Mr Howes says it is time to "hit the moment" like Labor did in 1975 when Prime Minister Gough Whitlam passed legislation outlawing racial discrimination.
"(It is) that moment when we send an unambiguous, liberating message rippling through society: that there is nothing inferior about gay people, there is nothing inferior about their relationships and our society does not condone anyone who believes otherwise.
"It is the Labor Party - and not individual Labor members - who need to take this overdue step forward."
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