Cowra man John Walsh, 79, pleaded guilty to murdering Frank Townsend in their Long Bay prison cell in the aged care unit for elderly and frail offenders in early January 2017.

In the NSW Supreme Court on Thursday, Justice Lucy McCallum - who previously sentenced him for murdering his wife and two grandchildren in 2008 - imposed another life sentence.

She noted his lack of remorse and the danger he still poses to others, despite being an old man.

"He is incapable of remorse," she said.

"He describes acts of murder as if he were explaining how to change a tyre."

In his police interview after he killed Townsend, Walsh described stepping forward with both hands, carrying the sandwich press in a pillow case, and "choom, onto his face".

"I don't work in anger, I work in tactical ... cold rage," he said.

In June 2008, Walsh stabbed his wife and bludgeoned her and his seven-year-old grandson with a hammer.

He drowned his five-year-old granddaughter in a bath at the Cowra home and also drowned the family dog.

Walsh's daughter came to collect the children and fought off his axe attack but suffered serious head injuries.

When asked what he was thinking when he hit Townsend, Walsh replied: "I don't think I was thinking anything."

"All I remember is bang, bang, bang ... I shut it out probably," he told police.

"The only thing I can't shut out is, my own family ... I don't know why that's happened because there was no anger, no drugs, no booze, just that depressing silence and I went and killed my wife.

"Maybe some people shouldn't be born."

Australian Associated Press