An academic study of Twitter opinions is pointing to a narrow defeat for the 'yes' vote in the same-sex marriage survey.

The study is at odds with myriad opinion polls showing the 'yes' vote could be as high as 60 per cent.

While 72 per cent of tweets favoured same-sex marriage, less than 15 per cent were sent by people over the age of 55, a Griffith University study shows.

Academics David Tuffley and Bela Stantic used advanced data analytics which they say have proven uncannily accurate at predicting the outcomes of hard-to-call polls, including the 2016 US presidential election.

They looked at the publicly available data from 458,565 tweets through October 2017.

On the face of it, when all the captured Tweets were considered, there appeared to be overwhelming support for 'yes', they said.

Taking the sentiment of the unique users into account, the adjusted figure in support came down to from 72 per to 57 per cent.

Adjusting the low proportion of tweets from over-55s and matching it against the 36 per cent of the voter pool they represent, support for 'yes' comes down to 49 per cent.

"So it is likely to be a close-run result, much closer than the earlier polls suggested, and leaning in the direction of 'no,' Professor Stantic and Mr Tuffley said.

"One of the problems with predicting poll outcomes is that people are often reluctant to say out loud what they really think about issues," the pair argue.

"What people say online can often be more accurate than what they say to each other in this age of political correctness."