The inmate charged with killing Waikato prison guard Jason Palmer today admitted his manslaughter.

Latu Savelio Halangingie Kepu, 21, punched Mr Palmer to the ground at Springhill Prison on May 15.

The 32-year-old father-of-two, later died in Middlemore Hospital with irreversible brain damage.

He was the first prison officer in New Zealand to be killed while on duty.

In the Hamilton District Court this morning, Kepu sat impassively in the dock handcuffed to a Corrections officer and surrounded by three others and a police officer.

He pleaded guilty to Mr Palmer's manslaughter and will be sentenced in the High Court on September 7.

As he was led away from court, the heavily tattooed prisoner briefly waved his hand to the public gallery, where some family members were believed to be present.

The Corrections Association of New Zealand is calling for an independent inquiry into Mr Palmer's death. Canz president Beven Hanlon last month said an independent investigation would answer guards' questions about the death of a colleague at the hands of a known violent prisoner.

Mr Hanlon said an increasing number of guards had been assaulted at prisons and staff deserved to be safe at work.

"This will only be achieved by an independent investigation into this incident giving us an unbiased determination of what lead up to and contributed to the death of corrections officer Palmer," he said.

The Minister of Corrections, Judith Collins, agreed with Mr Hanlon that it was important to find out exactly what happened on May 15.

But she has ruled out an independent inquiry, saying the full facts should emerge from investigations by the Department of Corrections, police, the Office of the Ombudsman and the Department of Labour.

Mr Hanlon said if there was nothing to hide, there should be no problem conducting an independent inquiry.

"Too many corrections officers are being assaulted every day and it has become an expectation.

"We will never learn from this tragedy if we don't shine a light on the decisions, policies and inactions that contributed to this death."

Last month Prime Minister John Key said the Corrections facility at Spring Hill was operating appropriately, despite the death of Mr Palmer.

Mr Key said it was the first time that a prison officer had been killed on the job in New Zealand and that indicated there was not a systematic problem.