The Australian Greens will seek to bring on a parliamentary vote on same-sex marriage "by hook or by crook" before the end of the year.
The minor party intends negotiating with Liberal senator Dean Smith on his private bill which emerged with consensus support from a Senate inquiry.
If the High Court rules against the government's planned postal survey, the Greens want to see Senator Smith's bill introduced to parliament next week.
"I think Dean's bill has the best chance of getting up so I think that would be the legislation we would want to see introduced," Greens senator Janet Rice said in Canberra on Tuesday.
"If that's the case, we could have marriage equality achieved next week. We could have weddings before Christmas."
The Greens wants to refocus their campaign on convincing Prime Minister Malcolm Turnbull to allow a free vote among Liberal MPs.
Leader Richard Di Natale said a successful High Court challenge would put enormous pressure on Malcolm Turnbull to allow a free vote in the parliament.
He said the Greens would ensure "that one way or another, by hook or by crook, we have marriage equality by the end of the year".
"Malcolm Turnbull better make sure there's a vote in this parliament or he won't have a job by the end of the year," Senator Di Natale said.
Nationals senator Matt Canavan, who opposes same-sex marriage, said the government must stick by its promise at the 2016 election to allow all Australians to have a say on the issue.
"I don't think we should change an act of parliament as important as the Marriage Act ... without a very considered debate," he told Sky News.
"I do think there is merit in letting all Australians have a say, particularly on my side of the debate."
The High Court case began in Melbourne on Tuesday, and a verbal decision could be delivered as early as Wednesday afternoon.
