A Rimutaka Prison guard's skull was fractured when he was "king-hit" from behind by a prisoner.
The attack happened in the Upper Hutt jail's high-medium security unit on Sunday afternoon. It is understood the prisoner was told to attack the guard by another prisoner.
Prison sources said the unit was understaffed and staff were reluctant to work overtime. Often this meant the unit was run "restricted", which meant only one side of the wing was allowed out at a time.
The unit was "run by the gangs", with members of the Nomads and Black Power on one side and the Mongrel Mob on the other, they said.
Staff had been too lenient towards the prisoners and things had spiralled out of control, a source said.
"They let them [the prisoners] spar and train martial arts and boxing in the yards, so of course the first people they use their new skills on when s... hits the fan is staff."
Prison staff were also scared of protecting themselves and restraining a prisoner because they feared they would get suspended or sacked, one source said.
In a statement, prison manager Richard Symonds said the guard was hurt after being struck in the head from behind in an "unprovoked attack".
He was checked by medical staff on site and taken to hospital for assessment before returning home. The guard would need follow-up treatment for his injuries.
Police were investigating.
"Despite the efforts we make to ensure risks are mitigated, there will be some occasions where assaults occur and, on occasion, staff sustain injuries," Mr Symonds said.
"Despite our sincerest intentions, we cannot prevent all assaults and no jurisdiction in the world has achieved this."
A Corrections Department spokesman said no comment would be made on the gang allegations because of "security reasons".
The assault comes days after Corrections Minister Anne Tolley announced an advisory panel would be formed to improve safety for staff within prisons.
She was concerned about a recent spate of attacks in jails and wanted to ensure everything was being done to minimise the risk.
In June, a Rimutaka Prison warden suffered serious head injuries when going to the aid of a prisoner who was being assaulted by a group of inmates.
A month later a Corrections officer was violently attacked at Hawke's Bay Prison, suffering a fractured eye socket and a deep cut to his cheek.
In September, another guard, at Hawke's Bay Prison, had bruises and swelling to his head and face after an attack by a prisoner.
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