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.