Prime Minister Malcolm Turnbull says he expects a plebiscite on same-sex marriage to be held by the end of 2016 if the Coalition is returned to government, his strongest commitment yet on the timing of the vote.

"We will hold it as soon as possible after the election. Given that the election is on 2 July, we do have ample time between then and the end of the year. So I would expect it to be held this calendar year," he said on Monday.

"But it will be held as soon as practical, as obviously legislation has got to pass through the Parliament so all I can do is give you my commitment to hold a plebiscite as soon as we can and it will be a very straightforward question and we will be asking the Australian people whether they support the definition of marriage being extended to include couples of the same sex."