Prime Minister Malcolm Turnbull has failed to endorse a pledge made by Attorney-General George Brandis to hold a same-sex marriage plebiscite, and change the Marriage Act, by the end of 2016.

Speaking on Sky News on Sunday, Senator Brandis said the government would hold a plebiscite on same sex marriage before the end of the year if it won the federal election expected late this year.

And if there was support for it, Senator Brandis said,the Coalition government would quickly introduce changes to the Marriage Act that he expected would become law "by the end of the year".

Mr Turnbull's office, however, distanced itself from Brandis' comments that a plebiscite would be held by the end of the year.