PUBLIC support for legalising gay marriage has dropped four per cent in the past fortnight, a new poll shows.

As the public debate over same-sex marriage heats up - with the 'No' campaign taking to skywriting their message over Sydney at the weekend - a Guardian Essential poll released today shows support for changing the law has dropped to 55 per cent in the past two weeks, down from 59 per cent

Meanwhile, the number of survey respondents who say they are opposed to gay marriage has risen to 34 per cent, up 3 per cent in a fortnight.

But despite the dip in support, the 'Yes' campaign looks set to succeed.

In the latest poll of 1808 people, 71 per cent of respondents who supported gay marriage said they would definitely vote, compared to 60 per cent of those who were opposed.

And among respondents who had already voted, 59 per cent supported same-sex marriage.

The campaigns for and against same-sex marriage will continue for another seven and a half weeks until voting officially closes on November 7.

The result of the postal survey will be announced at 11.30am on November 15.