A 77-year-old has been charged with murder after allegedly bashing to death his 71-year-old cellmate with a sandwich press in the frail aged unit at Sydney's Long Bay jail complex.

John Walsh is expected to appear at Parramatta Bail Court tomorrow.

He is already serving a life sentence for the murder of his wife and two grandchildren at Cowra in the state's central west in 2008.

Walsh was also sentenced for the attempted murder of his daughter who worked as a police officer.

A police spokesman said the assault at Long Bay happened before 11:00pm last night and the two men were cellmates.

They were housed in the Kevin Waller Unit, part of the Metropolitan Special Programs Centre at Long Bay Correctional Complex, in Matraville.

Sounds heard from the cell

A Corrective Services NSW spokeswoman said a correctional officer had heard sounds coming from the cell housing the two prisoners.

"The officer switched the cell light on and saw the 71-year-old had been assaulted," she said.

The two inmates were housed in Kevin Waller unit, which houses frail and elderly prisoners. ( ABC )

He was taken by ambulance to St Vincent's Hospital and died later, the spokeswoman said.

Police have seized items from the shared cell.

Following inquiries by the NSW Crime Command's Homicide Squad, Walsh was charged with murder.

Sandwich press used as weapon

New South Wales Corrective Services Minister David Elliott said there were three separate inquiries into the man's death.

Mr Elliott defended the presence of a sandwich press in the cell, which police allege was used as a weapon.

"It is not uncommon within the prison system, depending on the circumstances of their incarceration, for inmates to have appliances so they can cook for themselves. It saves the taxpayer money," he said.

A spokesperson for Corrective Services NSW said every death was treated seriously.

"All deaths in custody are investigated by the CSNSW Investigation Unit and the NSW Police, and a report is prepared for the NSW state coroner, who conducts an inquest into each case," they said.

There were 16 deaths in custody at NSW Correctional Centres in 2016: nine from presumed natural causes, five from apparent suicide, one from a presumed fall and one from an unknown cause.