Serious concerns have been raised for the wellbeing of an Indigenous man that has been in solitary confinement at a South Australian prison for more than two years.

That concern has grown since two Aboriginal advocates, including a former Aboriginal liaison officer at Yatala Labour Prison, spoke out this month after both were allegedly prevented from checking on prisoner Robert Barnes.

Mr Barnes was isolated after reportedly assaulting a corrections officer while serving an 11-year sentence for a range of offences in the high-security G Division of the corrections facility in Adelaide.

One of the state's prominent Aboriginal Elders, Uncle Major "Moogy" Sumner, a member of the South Australian First Nations Voice to Parliament, says he was denied access to visiting Mr Barnes after raising the inmate's predicament with the state advisory body.

"I was going to see him, then I got told that because I'm a member of the Voice in SA and I took it to the Voice, and they found out I mentioned it there, they said I could not go in and see him – but that's what the Voice is for," Sumner told the media during a protest.

"Anything wrong with Aboriginal people, we take it to the government."

The Ngarrindjeri man was set to meet the state's head of correctional services, David Brown, to discuss the issue over the lack of transparency.

"We'll talk about what we can do – we have to get (Barnes) out of there," Mr Sumner said.

"We should all be furious – Aboriginal people make up a record 31 per cent of the adult prison population."

Mr Sumner, who has long advocated politically for fairer treatment for Indigenous inmates, said Mr Barnes has "lot of problems" and would be "in a bad way mentally", and would culturally have benefitted from greater Aboriginal connection and interaction through visits.

"He's got problems because of how he was getting treated," Mr Sumner said.

Melanie Turner, who resigned as the prison's liaison officer over the incident, said earlier this month that the time Mr Barnes has spent in solitary is "getting close to 800 (days)".

Ms Turner said advocates had not heard of any prisoner – Indigenous or otherwise – being kept in solitary confinement for that long.

She last saw Mr Barnes at the end of March before quitting the post last month.

Ms Turner has claimed she was targeted and bullied inside the prison, and made to feel she had to resign from the role.

"I resigned because I was pushed out," she said.

In a letter to Correctional Services, addressed to Mr Brown, while still employed back in May, Ms Turner wrote she had been "hindered" in performing her role amid a ban on accessing G Division. She said it was "heartbreaking and soul-crushing" to be cut off from stopping Aboriginal deaths in custody.

Mr Barnes has reportedly self-harmed and attempted suicide twice. The most recent report has said Mr Barnes chewed his own little finger off down to a knuckle.

Ms Turner said Mr Barnes's sister told her he had tried to kill himself before the protest was held just outside of the prison walls.

She had advocated for Mr Sumner to initially visit Mr Barnes before he was allegedly denied.

"I said, 'Wow, I'll tell Major Sumner', and the (unnamed) person said, 'You can't tell him, and you can't tell the prisoner'," she said.

A spokesperson from the South Australian Department of Correctional Services denied Mr Sumner has been "banned" from the prison, though access to Mr Barnes's location is highly restricted for visitors.

"The safety of any visitor along with staff and prisoners is paramount when considering access, particularly to the state's high security prisons," the spokesperson said in a statement.

The Human Rights Law Centre has condemned solitary confinement as a "cruel practice that causes irreparable harm to the people who are subjected to this form of physical and sensory isolation", calling democratic on governments to ban the "archaic and inhumane" practice.

The centre also defines solitary confinement as "for 22 hours a day or more without meaningful human contact".

United Nations' own standards – often termed the Nelson Mandela rules – prohibit the practice for more than 15 consecutive days.

The matter has also grabbed the attention outside of the South Australian jurisdiction.

Federal Independent senator David Pocock of the ACT has urgently called for an "immediate investigation" into the conditions of the Yatala Labour prison, citing serious allegations of cruelty against the high-risk inmate while echoing broader concerns of preventable deaths within correctional facilities within the nation.

Senator Pocock has written to South Australian Minister for Correctional Services, Emily Bourke, to seek detailed facts about the placement and treatment of Mr Barnes.

"In view of these serious allegations and the clear threat and risk to (Robert) Barnes' health and wellbeing, I request that your government initiate an immediate investigation of these matters," Senator Pocock stated in the letter.