The most recent Resolve Political Monitor survey, published in Nine newspapers on Monday, showed 43 per cent of voters supported a plan to enshrine an Indigenous voice into the constitution, down 20 percentage points from a year ago.

The percentage of Australians in favour of the referendum has dropped for the fifth month in a row and since the last survey Victoria has flipped to a majority ‘no’ state, leaving Tasmania the only jurisdiction left in the ‘yes’ camp.

For the voice to succeed, the ‘yes’ campaign will require more than 50 per cent of the vote across the nation and in four of the six states.

South Australia along with Tasmania is considered a key swing state for the referendum, but Resolve polling last month showed that only 46 per cent of South Australians intended to vote yes.

Environment Minister Tanya Plibersek said there were still many undecided voters who could be convinced.

“We’re going to ask them to vote ‘yes’ because this acknowledges 65,000 years of Australian history,” she said on Monday.

“This idea came from Aboriginal people, well over 80 per cent of them support it. This is not a committee that has a veto over parliament. It doesn’t stop things happening.

“It is a committee to give advice, it really is a lot less scary than some of the ‘no’ campaign are making it out to be.”

However, critics of the referendum including Nationals MP Barnaby Joyce claim the voice lacks transparency.

“It will fundamentally change how this nation works and that is why people are moving away from it,” he said.

The news comes as postal voting applications for the Indigenous voice to parliament open, with the legal order to hold the referendum expected to be handed down on Monday.

Governor-General David Hurley will issue the writ compelling the Australian Electoral Commission to hold the October 14 poll, kick-starting its processes.

Postal vote applications will open on Monday once the legal document has been issued and will close a month later on October 11.

The electoral roll closes seven days after writs are issued – meaning Australians have a week to ensure they’re enrolled.

Voting in the referendum is compulsory, and failure to do so may result in fines.

Hundreds of early voting centres will be available from October 2, with centres to open in the ACT, New South Wales, Queensland, and South Australia a day later due to a public holiday.

Australians will be asked to vote on constitutional recognition of Indigenous people and to enshrine a new advisory body called the voice.

Indigenous leader Noel Pearson said the referendum “absolutely” has a chance of winning.

“I just don’t believe when the hand of friendship and reconciliation is extended from Indigenous people that at the end of the day, their love will be unrequited,” he said on Sunday.

Opposition Leader Peter Dutton has pledged to hold a second referendum if the upcoming vote fails, and should the coalition be returned to power.

-with AAP