Labor senator Penny Wong says the prime minister is Australia’s biggest roadblock to marriage equality.

She has also slammed the “illogical and outright offensive” language used by those against same sex marriage.

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“I wish the standard of this debate would perhaps be a little better than suggesting that marriage equality will somehow lead to polygamy,” she said.

Earlier Labor MP Joel Fitzgibbon said he’d changed his mind on gay marriage after voting against it three years ago and believed the reforms would pass parliament if Tony Abbott allowed a free vote.

“He’s using his power over the party room to represent the case of a shrinking minority over the case of a growing majority in the people’s house,” Mr Fitzgibbon told Sky News.

Government frontbencher Eric Abetz has defended penning a sternly-worded rebuke to Hobart City Council for expressing support for gay marriage.

In the letter, dated June 17, the Tasmanian senator said the council’s resolution “undermines the important social institution of marriage” and had nothing to do with the priorities of ratepayers.

Senator Abetz earlier suggested frontbenchers who wanted to cross the floor in parliament over the issue should resign, and that allowing gay marriage would open a “Pandora’s box” and potentially lead to polyamory.