Australians are voting overwhelmingly in favour of legalising same-sex marriage, according to a new poll that suggests turnout for the voluntary national postal survey will be high.

The ‘No’ campaign was quick to pour scorn on a Sky News/ReachTEL poll released on Monday afternoon that found ‘Yes’ held a commanding lead among the 79 per cent of respondents who said they had already voted.

Of those who said they had already mailed their ballot, 64.3 per cent had voted ‘Yes’ compared with 15.5 per cent for ‘No’.

With a sample size of nearly 5000, the survey is the largest since the campaign began.

The ‘No’ side has not taken the lead since the campaign began, though some surveys had previously suggested it was closing the gap.

Liberal MP Kevin Andrews was sceptical of the polling. Photo: AAP

The Coalition for Marriage campaign dismissed Monday’s poll outright.

Noting an earlier ReachTEL poll had very different findings, a Coalition for Marriage spokeswoman told The New Daily that of the two polls, “we prefer the second one”.

Asked to clarify, the spokeswoman said: “The two polls are conducted by the same company but so ridiculously incongruous.”

“How can either be credible?” she said.

That poll of 1000 respondents found 61 per cent of those who had already voted supported ‘Yes’, with 39 per cent backing ‘No’.

High turnout expected

Earlier on Monday, internal polling released by the ‘Yes’ campaign found 77 per cent of those surveyed had already voted.

The Newgate Research survey recorded a high youth turnout, with 69 per cent of respondents aged 18 to 24 having voted and another 17 per cent planning to participate.

The projections of a high turnout would appear to vindicate Prime Minister Malcolm Turnbull’s expression of confidence last week that there would be a “very high turnout, particularly for a voluntary postal vote”.

The Australian Bureau of Statistics will on Tuesday reveal how many ballots have been returned.

Commenting on the internal polling, Equality Campaign executive director Tiernan Brady said it showed Australians had seen through the “white noise and red herrings”.

“This poll is a reflection of the support marriage equality has in Australia, this support has existed for years, the majority of Australians want this done.”

‘No’ campaigner and Australian Christian Lobby managing director Lyle Shelton doubted the turnout was already as high as 77 per cent, though he said he wanted to see a strong participation rate.

“I suspect there are still millions and millions of Australians who are yet to cast their vote,” he told Sky News.

Mr Shelton described the campaign as a “David and Goliath” battle, saying the ‘No’ campaign would make its case “right down to the wire”.

Veteran Liberal MP Kevin Andrews, who is campaigning for a ‘No’ vote, said he was surprised by the ReachTEL poll results, particularly the high participation rate.

Some same-sex marriage advocates have expressed concern that turnout would be low for the voluntary survey, rendering the result unrepresentative of the electorate’s views.

Monday’s ReachTEL poll is likely to calm nerves among some same-sex marriage supporters including long-time gay rights campaigner Rodney Croome, who told The New Daily recently that the ‘Yes’ campaign needed to change tactics.

The poll also found nearly six per cent of respondents said they would not vote in the survey, while 2.8 per cent were yet to receive their ballot.

Another six per cent said they had not yet voted but planned to send back a ‘Yes’ ballot. A similar number of respondents (5.7 per cent) said they intended to send back a ‘No’ vote.

Australians are encouraged to return their postal survey by October 27, with a final deadline of November 7.

The result will be announced at 11am on November 15.