Australians have to vote 'Yes' or 'No' to a question asking whether they agree to alter the 122-year-old constitution to recognise Aboriginal and Torres Strait Island people, and create a body, called the Voice to Parliament, that can provide advice to the government.

More than 4 million people have already cast their ballot after early voting began on Oct. 2.

With less than two days to go before voting day on Oct. 14, those opposed to the proposal lead the 'Yes' camp by 56% to 38%, according to the final poll by YouGov published on Thursday. Some 6% of those polled were undecided. Yougov polled 1,519 voters for the survey.

"Our final poll indicates a sweeping ‘No’ victory – with nearing six in 10 voters intending to cast a ‘No’ vote," said Amir Daftari, YouGov Director of Polling and Academic research.

"Our detailed analysis indicates that it is very unlikely that 'Yes' will win anywhere apart from a number of inner metropolitan seats.”

'Yes' leads among the young while older voters overwhelmingly favour 'No', the polls found.

Australia's Indigenous citizens, about 3.8% of the country's 26 million population, have inhabited the land for about 60,000 years but are not mentioned in the constitution and track below national averages on most socio-economic measures.

Support for the referendum, the first since voters rejected a 1999 proposal to become a republic, has ebbed over the past few months. Supporters argue the Voice will bring progress for Indigenous Australians, while some opponents say enshrining one group in the constitution would be divisive.

Others against the Voice have described it as tokenism and toothless.

Referendums are difficult to pass in Australia, with only eight referendums being approved since it became a country in 1901.

Constitutional change requires a majority of votes nationwide and in at least four of the six states.