A Catholic archbishop says Australia needs to enshrine into law basic religious rights before making any moves to allow same-sex marriage.
Tasmania's Julian Porteous said simply providing exemptions for some people, as outlined in a government proposal, was not enough.
"It would be important that the government - before proceeding with anything to do with marriage - formally recognises that the right to religious freedom needs to be guaranteed and protected for all Australian citizens," he told a Senate hearing in Sydney on Tuesday.
The inquiry is looking into a draft bill legalising gay marriage that the government would introduce to parliament following voter approval.
It covers exemptions for ministers of religion, marriage celebrants and religious bodies and organisations to refuse to conduct, service or solemnise same-sex marriage, the impact on sex discrimination laws and what consequential changes to other laws would be needed.
Archbishop Porteous is concerned that if the bill is enacted, it would put pressure on Catholic institutes to promote same-sex marriage even though it goes against the tenets of their religion.
"It has vast consequences which will flow as time goes on and activists will be wanting to take things further and further and impose these views on society as a whole," he said.
"I fear that what will happen if things come to pass, it'll just be an ongoing battle. It'll set up antagonisms and bitterness. Nobody will be happy."
Archbishop Porteous made headlines in 2015 after authorising the distribution of an anti-gay marriage booklet.
The publication, titled 'Don't Mess With Marriage', was produced by the Australian Catholic Bishops' Conference and sent to thousands of Tasmanian families through the church's school system.
Rights activist Martine Delaney lodged a formal complaint with the state's anti-discrimination commissioner but later withdrew it.
She claimed it spread the message that messing with marriage equated to "messing with kids".
Archbishop Porteous said the text was misinterpreted and not intended to be derogatory, accusing Ms Delaney of "setting out" to be offended.
On Monday, the peak bodies for Australia's legal profession and civil celebrants said there was no need to extend exemptions that exist for ministers of religion to others.
The Law Council argued it would discriminate against same-sex, intersex and transgendered couples without any proper basis.
A third day of hearings will be held in Canberra on Wednesday.