The Australian Greens leader says if the party's bid to knock down the same-sex marriage vote fails in the High Court, the "yes" vote will still "win big".
Richard Di Natale believes the upcoming challenge will overturn the "expensive" and "unnecessary" postal vote, which he described as a "glorified opinion poll".
"However, should it go ahead, make no mistake. We are going to win, and win big," he told reporters outside the Bridal and Honeymoon Showcase in Melbourne on Saturday.
"We are going to make sure that if Malcolm Turnbull wants to hear what Australians think, well he'll hear it good and hard."
Greens volunteers spent the morning distributing "Vote Yes" leaflets to those at the wedding showcase.
Greens LGBTI spokesperson Janet Rice and lobby group Australian Marriage Equality are challenging the postal vote, claiming the government doesn't have the right to spend the money it has now allocated to the ballot.
They say the government also stretched the rules by enlisting the Australian Bureau of Statistics.
"As a litigant in the High Court in the challenge against this postal plebiscite, I'm optimistic of the fact the High Court is going to strike down the government's overreach on this issue," Senator Rice says.
