The Corrections Association claims a prisoner from Auckland Prison at Paremoremo was attacked by two other inmates.
The man was attacked yesterday in the maximum security block and suffered serious injuries.
The Department of Corrections is refusing to name the injured man but Radio New Zealand understands he is Nikki Roper, who is serving a life sentence for murdering his former girlfriend Alexsis Tovizi in 2010.
Corrections Association industrial officer Beven Hanlon said staff said it was one the most violent attacks they have seen at the prison.
"What they've told me is that two prisoners were jumping on this other prisoner's head and arms... You could clearly see that at least one arm was broken, due to the fact that it was bent in areas it shouldn't have been bent, and that he had significant head injuries."
Mr Hanlon said prison staff needed to be equipped with pepper spray to help stop violence.