The Turnbull government has declared the postal survey will be an authoritative verdict on same-sex marriage, after the Australian Bureau of Statistics revealed nearly 60 per cent of eligible Australians have already voted.

The bureau said it had received 9.2 million survey forms - or 57.5 per cent of the 16 million total - as of Friday, with five weeks to go before the deadline for responses to be received.

Although lower than the estimate in two recent polls, the turnout is widely viewed as substantial and in line with expectations. It already eclipses the 47 per cent turnout in a 1998 postal ballot on delegates to a constitutional convention, and is close to exceeding the 60.5 per cent turnout at the 2015 Irish referendum on same-sex marriage.

Acting Special Minister of State Mathias Cormann, who has responsibility for the survey, said the preliminary figures showed Australians were "enthusiastically embracing the opportunity to have their say" on same-sex marriage.