A Sydney serial killer, known as The Mutilator, has died aged 90 after becoming the longest-serving and oldest prisoner in New South Wales.

William Macdonald, was jailed for life in 1963 for murdering four men in Sydney and another in Brisbane.

Macdonald was widely regarded as Australia's first serial killer, and gained notoriety for slicing off his victims' genitals.

Born Allan Ginsberg in England in 1924, Macdonald would lure his victims into dark places before attacking them.

His victims were killed in 1961 and 1962 in inner-city areas of Sydney including Darlinghurst and Moore Park.

During his trial, some members of the jury reportedly fainted while hearing the gruesome details of his crimes.

Macdonald was being held in Sydney's Long Bay Jail before he was taken under guard to Sydney's Prince of Wales Hospital where he died of natural causes on Tuesday.

He was declared insane when convicted of the killings but was later found to be sane enough to be released back into society.

He reportedly chose to remain in prison.

In 2003, the Sunday Herald Sun quoted him as saying: "I have no desire to go and live on the outside. I wouldn't last five minutes".