Prime Minister Malcolm Turnbull is selling the postal vote on same-sex marriage as a chance for "all Australians to have their say" on the issue.

"It's an important question. Everyone will get an opportunity to have their say," he said on Thursday.

There is a lot riding on the postal plebiscite. The government says unless there is a majority "yes" vote, it will not allow a vote in parliament.

But will the non-compulsory $122 million vote accurately reflect what Australians think about marriage equality?