Those threats were never reported to Mr Palmer and Mr Matenga said the non-reporting played a "material part in the events that unfolded''.
Mr Palmer's mother, Ada, who lives in Virginia in the United States, said she had not been told the Coroner's report was due out today.
"But then everything that's happened in the courts, in the Coroner's court, everything I've learned the media has sent me a copy,'' she told Radio New Zealand.
"I swear to you I'm not bitter. I hold no angst towards anyone from New Zealand.''
She said it would have been nice to have been told about the inquest from the Coroner.
Ms Palmer said she believed part of the reason her son was killed was because he was a former marine from the United States.
"He was an in-your-face person from America ... and did not know how to blend with the culture of the prisoners and the Correction officers.''
She said the clash of the cultures was too great.
"He should have left New Zealand.''
An investigation by the then Department of Labour and an internal investigation by the Department of Corrections both concluded that no breach of policy or procedure had been identified.
But Coroner Matenga said he did not agree, given the threats made against Mr Palmer were never reported, and he made five recommendations for the department.
Those included developing a policy to deal with the transfer of prisoners and establishing a timeframe for a prisoner to be classified.
He also suggested Corrections consider adopting the alternative unlock procedure - whereby the prisoner is asked to stand at the back of the cell and turns to face the wall so they can be handcuffed - when dealing with maximum security prisoners who are being temporarily held in a non-maximum facility.
Department of Corrections' northern regional manager Gavin Dalziel said the findings were "very important'' but he would not say whether any of the specific recommended changes would be made.
"We will need to consider very carefully and closely before we decide what we actually do with them. At this stage it's really a matter of looking at what the Coroner is saying and taking the appropriate action from there.''