The government won’t take any further action on same-sex marriage if the High Court strikes down the government's national postal ballot.
Marriage equality activists are launching a High Court bid to head off the ballot, saying it breaches the constitution, and funding the vote exceeds the government's power.
Asked what would happen if the court struck down the ballot, Mr Turnbull told reporters: "Our policy is very clear. We will not facilitate the introduction of a private member's bill on this matter unless the Australian people have given their support through a 'yes' vote through this national vote."
Labor has begun encouraging voters to check their enrolment and get behind the 'yes' campaign, but shadow attorney-general Mark Dreyfus voiced fears the ballot would not have the same legal protections as an election.
Mr Dreyfus said campaign material would not have to be authorised - so voters could not tell who has sent it to them - and there was nothing to stop a person from filling in someone else's ballot paper.
There was also no way to question voting irregularities or dispute the result through the Court of Disputed Returns.
"It's more proof that a postal survey is an expensive, inaccurate and divisive waste of time," Mr Dreyfus said.
Some marriage equality advocates are weighing up a boycott of the ballot.
Former High Court judge Michael Kirby wants the plan abandoned, saying he was happy to wait to wed his partner of 50 years instead of having the public postal vote.
"I feel as a citizen I'm being treated in a second-class way," he said of the ballot.