A former worker at Rimutaka Prison has revealed Burton was regarded as unstable and potentially violent a month before he was granted parole.
Andy Coward, who has a PhD in cognitive psychology, was working in the segregation unit at Rimutaka Prison when Burton was transferred there about May last year.
The parole board decided to release Burton in March pending a psychological assessment. He was granted parole in June and released on July 10.
Dr Coward has worked in prison systems for 20 years across four continents and dealt with Burton for a fortnight. "If he [Burton] was suitable for parole, surely he would have been in a lower risk unit. You don't put him in a high security unit if he's going to be released. That doesn't make any sense whatsoever," Dr Coward said.
"I couldn't believe they let him out. He clearly wasn't stable."
Dr Coward said he had no issues with Burton and "consequently never saw the worst of him".
"I never saw him strike anyone, but I saw him lose his temper a few times. He certainly had a potential for violence."
The Corrections Department would not comment on why Burton was transferred nor whether he remained there until his release.
The department has generally refused to comment on Burton until its internal review of the case is completed.
President of the Howard League for Penal Reform Peter Williams, QC, questioned whether an inmate in the segregation unit would be ready for release.
"I would have thought he'd go through a period of lessened security in prison and that would continue - though it's not always practical - outside the prison so the adjustment to society is not abrupt, but gradual," Mr Williams said.
"If a person is regarded as being so difficult that he needs to be segregated in prison, one would think it would be unlikely that he would immediately adjust."