A bill recognising overseas same-sex marriages has been overwhelmingly voted down in the Senate, with Government and Opposition senators crossing the floor to vote.

Last year the Senate voted down a bill aimed at recognising the marriages of gay and lesbian Australians who have been wed overseas, and the Coalition, which officially opposes gay marriage, labelled today's bill as a backdoor way of reigniting the debate.

But Liberal-National Senator Sue Boyce disagreed and crossed the floor to support the bill.

"Within the Liberal Party one always has the option of exercising a free vote, and if we are to vote on this legislation, I will be voting for this bill because I think it advances the cause of same-sex marriage in Australia," the Queensland senator said.

"Same-sex marriage is not going to be the end of the world for anybody - especially not for children or for couples in Australia."

Senator Boyce indicated before the vote that she would cross the floor against her party's position.

Labor senators also split on gay marriage recognition

There was also disagreement in Labor ranks, with 10 Government senators, including ministers Stephen Conroy, Don Farrell and Jacinta Collins, splitting from the majority of their colleagues to vote against the bill.

Penny Wong was one of those Labor senators voting for the bill.

"If you subscribe to the principle of equality - and I am sure that most in this chamber would say that they do - then simply substitute words such as interracial or different age in lieu of same-sex in this debate and see if it changes your view," she said.

Same-sex marriage is legal in: New Zealand, 2013

New Zealand, 2013 Uruguay, 2013

Uruguay, 2013 Denmark, 2012

Denmark, 2012 Argentina, 2010

Argentina, 2010 Portugal, 2010

Portugal, 2010 Iceland, 2010

Iceland, 2010 Sweden, 2009

Sweden, 2009 Norway, 2009

Norway, 2009 South Africa, 2006

South Africa, 2006 Spain, 2005

Spain, 2005 Canada, 2005

Canada, 2005 Belgium, 2003

Belgium, 2003 The Netherlands, 2000

"Can you imagine if anyone in this Parliament actually asserted that a couple in Australia could not get married because the person that they loved, one of them, was of a different skin colour?

"These notions are not only anachronistic, they are in fact offensive."

In the end the bill was defeated, with 44 votes against and 28 in favour.

Opposition Leader Tony Abbott says he understands Senator Boyce's decision to cross the floor.

"I appreciate Sue's position. We're not a Stalinist party, never have been, never will be. Obviously we have a clear position that we don't support gay marriage as a party," he said.

Earlier this year, the Federal Government sought to remove a discriminatory hurdle against same-sex couples who want to marry overseas by introducing a certificate of no impediment.

The document proves a person is over 18, is not married and there is no other barrier to them getting married.

Before that, the Government only provided the certificate for heterosexual couples, but same-sex couples have been able to apply for the document since February this year.