Pauline Hanson has warned polygamy and underage marriage could be among the unintended consequences of legalising same-sex marriage.
Despite raising concerns, the One Nation leader said she is still unsure whether she will support the bill before the Senate and will give her party colleagues a free vote on the issue.
"My concern is that, in time to come, the parliament and its members could at any time change this (definition) to include multiple marriages or marriages of people under a certain age," Senator Hanson told parliament on Tuesday.
Senator Hanson will move an amendment to the bill to protect civil celebrants who want the right to refuse to marry gay couples.
"I don't believe we are fully aware of the ramifications this is going to have on our society," she said.
She believes there should have been a referendum to define the meaning of marriage in the constitution, rather than the voluntary postal survey that returned a 61.6 per cent 'yes' vote.
Senator Hanson used the 1967 referendum over recognising Indigenous Australians as an example of a change which had created unintended consequences like "reverse racism".
"We have now made laws that give Aboriginals more rights than other Australians," Senator Hanson said.
