American-born Jason Palmer, 33, died in Auckland's Middlemore Hospital yesterday after being punched by an inmate at Spring Hill Prison at Hampton Downs, about 65km south of Auckland, on Saturday afternoon.
Department of Corrections chief executive Barry Matthews said it appeared Mr Palmer was punched shortly after he and two other prison guards opened the inmate's cell.
He fell back and hit his head on the floor as the other two guards restrained the prisoner.
Although there had not yet been an autopsy, Mr Matthews said it appeared a combination of the punch and the striking of his head on the floor caused Mr Palmer's fatal injuries.
The inmate, reportedly a Killer Beez gang member, was due to appear in Hamilton District Court today charged with assault. Further charges were expected to be laid.
Mr Palmer, a former United States Marine Corps member, moved to New Zealand eight years ago and lived with his wife and two children, aged five and two.
His family said they were focusing on remembering him as he was, rather than on how he died.
Younger brother Justin Palmer, 28, speaking from his mother's house in Woodbridge, Virginia, told the Dominion Post his brother had lived for his wife and children.
"He was a dedicated family man -- he went to work to support his family," he told The Dominion Post.
"He would make you laugh -- even during a very dark time, even being down in the dumps."
Mr Palmer's death sparked calls from the Corrections Association for better protection for officers in prisons.
Association president Beven Hanlon said police dealt with the same people but were much better protected, with equipment including dogs, stab-proof vests and Tasers.
However, Mr Matthews said he believed the department took the security of its staff very seriously, but there was always a risk given that the prisons were full of violent people.
"We've had a staff safety programme to actually improve safety. We've increased the equipment, we've put a lot of effort into training," he said.
"But at the end of it there's always been that potential risk that somebody just impulsively attacks an officer."
Mr Matthews said there was nothing to indicate that what the officers did was anything other than proper procedure.
Corrections Minister Judith Collins said yesterday she did not wish to comment on criticisms of the general safety of the staff while investigations were under way.
