Malcolm Turnbull is expected to come under intense pressure to pass legislation extending full marriage rights to same sex couples before Christmas as Australians vote 6 to 4 in favour of changing the law, according to consolidated opinion polling.

With just days left to mail in returns to the protracted survey, analysis of the combined public and private opinion data available between August 1 and October 22 reveals the "yes" case has a strong lead over its negative alternative, with Australians also returning the survey forms in enormous numbers.

Number-crunching by one of Australia's most experienced public opinion pollsters, John Stirton, suggests there is now very little chance of the "no" case reaching a majority, "unless there is something terribly wrong with the polling, the postal survey, or both".

Mr Stirton said the postal survey, in which the Australian Bureau of Statistics estimates that 74.5 per cent of eligible Australians have voted so far (a more accurate finding derived from scanning envelopes rather than merely weighing them), had achieved much higher levels of public engagement than its critics predicted, leading to a strong "yes" vote.

The growing likelihood of a clear "yes" verdict on November 15 puts the pro-change Prime Minister on a collision course with religious conservatives in his party room. They are expected to move to a parliamentary 'plan-B' if the estimated $122 million voluntary postal survey goes against them.

Bracing for defeat, they will step up the arguments for "free speech" and "religious protections", both of which are seen by same-sex marriage advocates as code for legalised discrimination against gay couples and for the advocacy of sexuality-based discrimination in religious schools and churches. Read more via Sydney Morning Herald