Michael Guider, the man convicted over the death of nine-year-old Bondi girl Samantha Knight, has died aged 73.
Corrective Services NSW confirmed Guider had died about 7:25am on Saturday morning.
"A 73-year-old inmate has died at Prince of Wales hospital," a spokeswoman said in a statement.
"As a matter of protocol, Corrective Services NSW and NSW Police investigate all deaths in custody regardless of the circumstances."
The 73-year-old was sentenced over the death of Samantha Knight in 2002. (AAP: Mick Tsikas)
Samantha Knight went missing in 1986 and, despite extensive searches, her body has never been found.
A judge found that Guider took Samantha to a location and gave her sleeping medication, most probably with the intention of taking indecent photographs of her.
After she stopped breathing, Guider disposed of her body.
He was given a 17-year sentence in 2002 after pleading guilty to manslaughter, and was refused parole several times.
Despite extensive searches, Samantha's body has never been found. (Supplied: Missing Persons Registry)
Guider was also found to have abused 13 other boys and girls, often drugging them in the process.
In 2022 he was re-incarcerated after police found child abuse material on his mobile phone.
Prominent child protection advocate, and founder of the Bravehearts charity, Hetty Johnston, said the news would be a relief for Samantha Knight's family and other survivors.
"For their whole lives, that just never leaves you, but maybe a little bit of closure in that he's no longer with us and no longer able to hurt anybody," Ms Johnston said.
"This is a man, as you know he's a serial child sex predator, who has caused enormous harm and has committed murder, and the families still have a life sentence for that."
Ms Johnston said the debate around his release sparked new laws enabling extended detention for sex offenders.
"That's why I lobbied for and won the Dangerous Prisoners Sexual Offenders Act.
"Whereby, when a sex offender commits serious offences, they can be detained past their release date if they're considered to pose an ongoing high risk."