Grandfather killer John Walsh has been accused of the murder of his 71-year-old Long Bay Prison cellmate.
NSW Police have confirmed the fatal attack, which happened just before 11pm on Monday.
"A man aged is his 70s is assisting police with their inquiries following the death of a second man aged in his 70s at a Malabar correctional facility overnight," NSW police said.
The Daily Telegraph understands NSW Corrective Service officers found the body of convicted killer Frank Townsend after allegedly hearing noises from one of the cells.
It is believed Walsh, 77, bashed Townsend to death with a sandwich press. There was no known animosity or history between the cellmates.
Wicked in the extreme
Walsh is serving a life sentence after pleading guilty in July 2009 to murdering his wife, Jean Walsh, 52, and two grandchildren, Jaime, 5, and Kevin, 7.
He also admitted trying to murder his daughter, Shelly Walsh, who was a NSW police officer at the time.
The mother of the children made the horrific discovery of her murdered family when she returned home to her parent's Cowra home in June 2008.
She had left her parents to care for her children while she worked an overnight shift at Parkes police station.
Walsh tried to attack his daughter with an axe and said, "I am doing this because I love you. When I am done with you lot I am going to Newcastle to kill your ex-husband. We are all better off this way. This is the way it has to be."
Walsh later confessed to police that he killed his wife by hitting her on the head about three times with a hammer shaft he called "Fred", before stabbing her with a knife and hitting her on the back of the head with a lump hammer.
When handing Walsh a life sentence in 2009 Justice Lucy McCallum said a "disturbing feature" of the case was that Walsh had never offered an explanation for the murders.
At the time Justice McCallum said the murders of his grandchildren fell within the worst case category and said "his acts were wicked in the extreme".
"The offender killed his young grandchildren when they had been entrusted to his care. He intended to kill them and planned their murders with grim attention," she said.
"He knew their mother was many miles away, trusting that her children were in safe hands. He abused the children's trust in him by coaxing them out of their beds."
She said the children suffered "a level of terror no child should know".