The Turnbull government will hold a people's vote on same-sex marriage by November, almost certainly in the form of a $122 million postal vote, with a view to resolving the issue by the end of the year.

The government will ask the Senate to approve a compulsory plebiscite on November 25, but in the likely event that is rejected, it will hold a postal vote with a November deadline.

The ballot would be voluntary, and conducted by the Australian Bureau of Statistics between September 12 and November, with a result to be declared by November 15.

If Australians return a majority "Yes" vote, the government will facilitate debate on a bill to legalise same-sex marriage before the end of the year, but MPs will not be bound to respect the will of the people.