Triple murderer and rapist John Ernest Cribb died at Goulburn jail on Wednesday.
Cribb, 67, was serving three life sentences for the stabbing to death three members of the Connell family in Baulkham Hills in north-west Sydney four decades ago.
John Cribb was serving three life sentences.
He reported feeling unwell earlier this week before he was found dead at the prison about 10.30am on Wednesday.
Cribb was on parole for armed robbery when he broke into the Connells' home and kidnapped Valda Connell, 39, and two of her six children, Sally, 10, and Damien, 4, in August 1978.
Their bodies were found in a car boot near Swansea. Cribb had raped Valda and then killed the woman and her children with a knife.
Cribb escaped with well-known criminal William John Munday from the Morisset Hospital for the criminally insane in April 1979, where he was awaiting trial for the murders.
They became the most wanted men in the state.
The pair committed 31 offences between them including rapes, kidnaps, assaults and robberies in Newcastle and Sydney before they were caught a couple of weeks later.
The Corrective Services Investigation Unit from NSW Police is investigating Cribb's death and will prepare a report.
All deaths in custody are referred to the NSW Coroner.