The Greens have likened the upcoming by-election in the South Australian seat of Mayo to "groundhog day", saying it will again be fought over the River Murray, and have revealed Ngarrindjeri elder Major 'Moogy' Sumner as their candidate.

Incumbent Rebekha Sharkie announced her resignation in the wake of the High Court's decision over Labor senator Katy Gallagher's eligibility, but intends to recontest the seat.

Georgina Downer has been endorsed unopposed as the candidate for the Liberal Party in the seat her father held for 24 years, until 2008.

Mr Sumner said his people had been caretakers of the land for a long time, and that is what has inspired him to put his name forward.

"I followed the Greens, watching what they're doing over the years, and I felt what they're doing is what we've been doing for a long time," he said.

"So why not go out there and join and be a part of it and be heard?

"This is the only earth we've got, even though people are trying to get to Mars. This is the one that we should be looking after."

He has a long connection to the seat of Mayo and has vowed to focus on environmental issues including the River Murray as part of his campaign.

"I come from there, that's my traditional land," he said.

"I was born there in Point McLeay Aboriginal community, I went to school there, I've seen the changes over time — not just with the community but with the land, with the lake."

Major Sumner performing with the Tal-Kin-Jeri dance group. ( ABC News: Michael Coggan )

The Greens regard this by-election as a re-run of the 2008 by-election, when the state of the Murray was a significant issue.

"It feels a little bit like groundhog day — it's back. And yet we seem to have not learnt," Senator Sarah Hanson-Young said.

At that by-election, Labor opted not run a candidate and the Greens got close to causing a major upset.

In 2014, Mr Sumner was made a Member of the Order of Australia for his services to the Indigenous community.

He formed the Tal-Kin-Jeri dance group and is well known as a performer and cultural ambassador.

Murray debate about more than the states, Sumner says

Senator Hanson-Young said she thinks Mr Sumner represents the Adelaide Hills electorate better than anyone else possibly could.

"Moogy and his family have been looking after this area for generations — for hundreds of years, for thousands of years — and it's time that the rest of us learnt a few things," she said.

"[He] is a passionate advocate for the health of our land and waterways, and dedicated to sharing his cultural values through his work in schools and the community."

Senator Hanson-Young praised Major Sumner's advocacy for environmental causes. ( ABC News: Isadora Bogle )

She said, if elected, Mr Sumner would be South Australia's first Indigenous federal MP.

Mr Sumner said his priority would be to look after the environment.

But he said securing the future of the River Murray was about much more than a fight between the upstream states and SA.

"Before we do anything else about anything round her, we make sure that that water's right first," he said.

"We need the water down here — fair enough, but the whole system needs to have that water going out through the mouth, otherwise later on down the track it'll just become a stagnant pool and it'll be poison."