The Australian Bureau of Statistics has begun work on the $122 million voluntary poll, with ballot papers due to be posted by September 12, completed by November 7 and a result to be announced on November 15.
A private member's bill would then go to parliament by the end of the year.
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.
Nationals MP Andrew Broad, who has threatened to quit the government if the coalition changes its plebiscite policy, is pleased Australians can have their say.
Asked about a possible boycott, the MP told reporters: "There were people who chose not to vote for Donald Trump because they walked away from it, and they got Donald Trump didn't they?"
Labor frontbencher Penny Wong, who made an emotion-charged plea during a speech in parliament on Wednesday for 'no' campaigners to leave children out of the debate, is hoping good-hearted Australians prevail.
Treasurer Scott Morrison defended the $122 million price tag of the postal ballot, insisting "keeping promises is money well spent".
