Former Liberal MP Pat Farmer urges older Australians to back Voice

"The simple fact of this matter is that the First Nations people, the first people that were here in this country before many of us ever got here ... they deserve a voice about their own futures," Mr Farmer said.

The former Liberal MP is running 14,000 kilometres around the country to spread a message of support for an Indigenous Voice to Parliament.

On Monday he made a stop in Canberra, where he was joined by Labor MP Andrew Leigh, Indigenous Marathon Foundation founder Rob de Castella and former ACT senator Gary Humphries for an extra five-kilometre loop around Lake Burley Griffin.

The 61-year-old said many young people "already get it" and urged older Australians "not leave it up to our kids to fix the problems that we can fix right now".

"I'm appealing to the people that are my age, and that's why I'm out here, that's why I put the shoes back on and that's why I'm running around Australia, because the young ones already get it," he said.

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"If we don't say 'yes' on referendum day, then it will never come up again in my lifetime or in our lifetime."

While a number of surveys have shown that support for the Voice has been falling, an Australia Institute poll found that 73 per cent of young people support the proposal.

Australians will vote on whether to enshrine an Indigenous advisory body in the constitution later this year, with Prime Minister Anthony Albanese set to announce the referendum date on Wednesday.

Mr Farmer said the Voice would be a "good thing" for the entire country.

"As Jim Chalmers, the Treasurer, noted, when I was running with him up on the Gold Coast, it will actually save the Commonwealth an incredible amount of money, which means it saves the taxpayers money, which means the whole country progresses," he said.

Dr Leigh said supporting Voice meant listening to First Nations people and recognising that they "should have a place in the Australian constitution".

"When we listen to First Nations people, we come up with better decisions. It's as simple as that," he said.

Mr Humphries also urged Canberrans to "get behind this referendum".

"Vote 'yes', put ourselves in a position where we have a constructive dialogue to promote the interests of Indigenous Australians and take forward the support," he said.