A grandfather who murdered his wife and two grandchildren in central NSW is being questioned by police after his cellmate was bashed to death inside Sydney's Long Bay Correctional Centre.
John Walsh, 77, is being interviewed over the fatal attack on convicted killer Frank Townsend, 72, at 11pm on Monday.
Townsend was allegedly bashed to death with a sandwich press, but a motive for the attack has not yet been established.
The incident took place inside the Kevin Waller Unit for aged and frail inmates of the jail, located at Matraville in Sydney's south-east.
Walsh is serving life in prison for the June 2008 murders of his wife, Jean, 52, his seven-year-old grandson, Kevin, and his five-year-old granddaughter, Jaime.
He bludgeoned his wife with a piece of wood and hammer and stabbed her in the neck, then battered Kevin to death and drowned Jamie in the bath inside their home at Cowra.
The children's mother, policewoman Shelly Walsh, found the three after she returned from a court appearance at Parkes. She was admitted to hospital soon afterwards to undergo surgery for an axe wound.
Townsend served time for manslaughter over the Port Macquarie death of Belinda Trad. Ms Trad's body was found with several gunshot wounds at a house in July 2010.
He was released in 2016 but had recently breached his parole conditions and had been taken back into custody
A NSW Police spokeswoman confirmed Townsend died "after an alleged assault".
It's understood he was unconscious when staff at the correctional complex found him and called triple zero.
The police spokeswoman said no further details were immediately available. No charges had been laid early on Tuesday morning.