The Nick Xenophon Team says a plebiscite is unnecessary, and parliament should make a decision on the issue.
The Greens, too, have vowed to block the plebiscite.
Senator Xenophon told reporters in Canberra his party's position is clear.
"MPs and senators are paid to make decisions, and this is a decision that should be made by a parliament in a free vote, rather than spending $160-million on a plebiscite. We think there are better ways that taxpayers' money could be spent on what effectively would be a non-binding vote, something that could be disregarded by the parliament, in any event."
The federal opposition has reiterated its call to examine the government's legislation for a same-sex marriage plebiscite before stating whether it will support it.
Labor has expressed concerns about the cost of a national vote, and the potential effects it could have on the L-G-B-T-I community.
It says the most effective way to achieve marriage equality is a direct vote in parliament, and has flagged plans to introduce a private members' bill to make that happen.
Marriage-equality spokeswoman Terri Butler told the ABC it's only fair the opposition see the plebiscite legislation before being asked to back it.
"This is a situation where the government has been talking about this plebiscite idea for more than a year - it was actually (former prime minister) Tony Abbott who first came out of that six-hour party room meeting and said, 'Oh, maybe a plebiscite' - and yet there's not been any sort of publication of what the question will be, or when the timing is going to occur, or what's going to be in the bill. There've been some leaks, but they haven't been confirmed, and so I think as a matter of prudence and good faith we would say, 'show us the bill and then ask us to express an opinion on the bill.'"
