Jason Palmer

A Waikato prison inmate was awaiting transfer to a high security prison at the time of a fatal attack on a prison guard, the Department of Corrections says.

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 segregated 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 Matthews said the inmate had been due to be transferred to a high security prison today.

"On Saturday morning he was reclassified because of a series of misconducts, increasing to maximum security category," Mr Matthews told Radio New Zealand today.

"We had arranged for him to be transferred on Monday morning to Auckland East Prison, that's Paremoremo, and that's why we had three officers doing the unlock, because of that classification and because of the fact that he was on directive segregation."

Prime Minister John Key told Newstalk ZB today that the death was a tragedy and he offered the family his condolences.

He said the prison service and police would investigate.

"Although on the advice I've had so far it looks as if procedure has been followed."

Claims the prison was overcrowded and the prisoner was doublebunked were incorrect.

"What's gone wrong is going to be a matter for the prison service and ultimately the police in their investigations."

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 newspaper.

"He would make you laugh -- even during a very dark time, even being down in the dumps."