The man accused of punching a prison guard in the face before he died is now charged with his manslaughter.

United States-born Jason Palmer, 33, was six months into the job when he was allegedly punched to the floor by a high-risk prisoner at Waikato's medium-security Spring Hill Prison in May. The father-of-two suffered irreversible brain damage and later died.

The 21 year-old inmate had been charged with assault but the charges were upgraded to manslaughter today.

He will appear in the District Court in Hamilton tomorrow. It is understood the Auckland man, who has since been transferred to the maximum security unit at Paremoremo, was a member of the Killer Beez gang.

Mr Palmer, a former US Marine, is survived by his Kiwi wife Tracy and their two children Riley and Abby.