Support for same-sex marriage has crashed ahead of the Turnbull government's postal survey, and only two-thirds of voters are inclined to take part, according to the latest polling from same-sex marriage advocates.

At the start of a two-month campaign, the confidential research provided to Fairfax Media shows support for a "no" vote has risen, as has the number of people who say they don't know how they will vote.

And alarmingly for "yes" campaigners, turnout could be very low, with just 65 per cent of voters rating themselves as very likely to participate – falling to 58 per cent among those aged 18 to 34.

It prompted a concerned campaign veteran to declare: "There is every chance we can slip behind and lose this."