"The only thing lacking here is Malcolm Turnbull's courage," Labor's Senate leader Penny Wong has declared, pushing back against criticisms from "so-called moderates" in the Coalition towards the opposition for challenging the same-sex marriage plebiscite legislation.

The Greens and Nick Xenophon Team have announced they will vote against it and Labor has hardened its rhetoric, reiterating arguments that a national vote will cost at least $160 million, fuel hurtful debates and won't bind Coalition MPs to vote for the result.

"The plebiscite is a political deceit imposed upon the Coalition by Tony Abbott and the conservatives. It would be expensive, divisive and it is non-binding," Senator Wong told ABC radio.

"It is a deceit that Malcolm Turnbull doesn't agree with. We know that because, before he became prime minister, he argued against it. Now what has happened is he lacks political courage.