In 1960s Sydney, William Macdonald was known as the "Mutilator", the villain who killed four homeless men and cut off their genitals with a hunting knife.

Much later, at Long Bay Correctional Complex, he was known as "Old Bill", the fitness freak who loved the news and Mozart.

At the time of his death on Tuesday morning, he was the "longest serving and oldest current inmate in NSW".

Macdonald, 90, terrorised Sydney from 1961 to 1962, striking in The Domain, Moore Park, Darlinghurst and Concord. He kept the parts of one of his victims in a bag, then threw it off the Sydney Harbour Bridge.

"It's difficult now to imagine the level of panic in the city," said Joe Payne, a reporter who covered the case for the now defunct Sun​ newspaper.

"Homeless men were fighting to get into hostels."

Hyde Park and The Domain were cleared of rough sleepers; policemen dressed up in torn, dirty clothes, trying to entrap their man.

Eventually, Macdonald was caught when railway porters in Melbourne recognised him from an Identikit image.

Despite testimony from three psychiatrists at his trial that he was insane, he was convicted of murder. He confessed in a 100-page document to all four killings, and to strangling a fifth victim, a Brisbane man, before his Sydney spree.

Born Allan Ginsburg, Macdonald said he had been mistreated by his father, who tried to toughen him up and make him smoke.

A schizophrenic, he said voices convinced him his victims were in fact "the Corporal", a British military officer who had raped him in the late 1940s.

Payne became the loner's first visitor in nearly 30 years of incarceration when he went to Cessnock Correctional Centre in 1992.

"Always, always that look, and the teeth coming out, and the eyes," said Payne, recalling the features that helped identify the murderer. In a word: "Frightening."

Writing for the Herald, Payne was forbidden from paying his subject for an interview so he offered a different gift: $100 worth of classical music cassettes.

Bach, Beethoven, but especially Mozart entranced Macdonald, who played the jail's piano when no one else was around. As a child, he had wanted to become a musician.

More than two decades after their first meeting, Payne visited the killer again, on his 90th birthday.

"He was a fitness fanatic and tremendously strong," Payne said. "He looked well for 90. He said he wanted to be the first prisoner in Australia to live to 100. He was happy."

He died of "apparent natural causes" at 8.20am on Tuesday in a secure annex of the Prince of Wales Hospital, a Corrective Services NSW spokesman confirmed.

Macdonald never smoked and had given up meat. Keeping fit, eating healthily, watching the news and ABC murder mysteries: these were the old prisoner's pleasures. Not that he thought he deserved them.

Payne remembered Macdonald saying he should be hanged along with Ivan Milat, the backpacker murderer, and whoever killed the nurse Anita Cobby.

"He never wanted to get out because he knew he would do it again."