Opposition Leader Bill Shorten will pressure the Prime Minister to allow a free parliamentary vote on same-sex marriage if the government's planned postal survey fails a legal challenge.
The High Court will rule on the challenge later on Thursday during question time in federal parliament.
Prime Minister Malcolm Turnbull says he is confident the postal vote will not be stopped by the court.
"Our advice has been such that we're confident the action we're taking is not going to be stopped or injuncted by the High Court," he told Nine's Today Show on Thursday.
But if the challenge is dismissed Labor will continue to campaign energetically for the 'yes' case, Mr Shorten insisted.
In the event it is ruled invalid the Labor leader will call on Mr Turnbull to immediately allow a free vote of all MPs.
"If Mr Turnbull won't or can't allow a full free vote in the parliament, if we are elected at the next election we'll just get on with it," Mr Shorten told reporters in Canberra.
Treasurer Scott Morrison would not be drawn on the government's next plan should the High Court rule against the postal survey, having already been prevented from holding a formal plebiscite.
"Let's see what the High Court decides," he told reporters.
But other government MPs, including former prime minister Tony Abbott, are more forthcoming.
"The prime minister should do what the prime minister is committed to do and that is that there won't be any parliamentary vote without a plebiscite first," he told 2GB radio.
That position is supported veteran Liberal senator Eric Abetz.
"It's a plebiscite or nothing," he told Sky News.
