Prime Minister Malcolm Turnbull has fuelled speculation the government will bypass Parliament and commission a postal vote on same-sex marriage, refusing to rule it out on Wednesday as the idea builds momentum among colleagues.

But it has been revealed Mr Turnbull argued vociferously against a postal vote when he led the campaign for an Australian republic, saying such a method "flies in the face of Australian democratic values".

The postal poll - a voluntary, non-binding vote conducted by mail - would substitute for the compulsory plebiscite, which failed to get through the Senate last year.

It is believed by some MPs that unlike a plebiscite, the postal vote would not need to pass the Parliament, and could be funded by the existing Commonwealth Electoral Act. The government is understood to be seeking advice on the logistics and viability.