"Each of us is more than the worst thing we've ever done."

It is a kind of compassion Indonesia's president has been unwilling to grant the condemned Bali Nine pair Andrew Chan and Myuran Sukumaran, but Stevenson is convinced Australians have every right to demand.

"We're all good at seeing each other's faults and not as good as seeing our own," Stevenson said from his office in the southern United States state of Alabama before a trip to Australia  this month.

"Which is why I don't think there is anything inappropriate about Australia telling America 'your system is messed up' or America saying to Indonesia 'your system is messed up'.

"Often times when the people who are at risk have zero political power … it takes a just society to stand in for them and demand more fairness."

Stevenson has won praise for his work tackling prejudice and poverty in US prisons, and has been hailed as "America's young Nelson Mandela" by South Africa's Desmond Tutu.

As a young lawyer in the 1980s he founded a legal practice dedicated to defending the poor.

"Most people who do horrible things have had horrible things done to them", he said. Determining culpability was as important to understanding guilt in a crime.

He is dismayed by the growing size and cost of the US's prison population, and startling evidence of racial discrimination, including that one in every three black male babies born this century is expected to be incarcerated.

"What is particularly shocking is that wasn't true in the 20th century or 19th century, but became true in the 21st century," he said.

But Stevenson maintains special focus on death penalty cases, a punishment he said could be popular but majority support did not make it legitimate.

His newly published memoir, Just Mercy, exposes faults in the US criminal system that has led to about 150 exonerations of condemned prisoners, including his clients.

The punishment is also heavily skewed to the poor, minorities and disabled.

Georgia, another US southern state, executed this week a 54-year-old man whose lawyer said had the cognitive and emotional functioning of an 11-year-old.

"There is really no circumstances where the death penalty is legitimate," Stevenson said.

"It is the perfect punishment that requires a perfect system, and I don't think we are ever going to have a perfect system."

Australians had an obligation to speak up for clemency for the condemned Bali Nine pair and not allow indifference to compromise human rights, he said.

"They might not be able to exercise control over whether these people get executed or not – they don't have that kind of power – but they can exercise control over their relationship to that country," he said.

Bryan Stevenson will appear at ANU in Canberra on February 18, at Melbourne Law School in Parkville on February 19, and will deliver the closing address of the Perth Writer's Festival.