The country's only privately-run prison, Mt Eden Corrections Facility, has had more prisoner assaults than any other jail for the last three years.
The prison's operations were contracted out by the Government and it has been run by global outsourcing firm Serco since May 2011.
Figures obtained by Radio New Zealand show that in the 12 months to June there were 139 fights between prisoners and 26 assaults on guards.
There were 141 prisoner assaults in 2013 and 149 in 2012. There were also 39 assaults on guards in the 12 months to June 2013, and 22 attacks in 2012.
Mt Eden has the capacity to house 966 prisoners. By comparison, Christchurch Prison has 926 inmates and reported 83 assaults last year.
The Corrections Association and the Public Service Association, which both represent prison officers working at Mt Eden prison, said the figures proved what they had long been saying: that there were simply not enough staff to keep themselves and inmates safe.
Public prisons are required to have at least least one guard for every 15 remand prisoners, with a typical wing housing 45 inmates.
However, Corrections Association president Beven Hanlon said there was no such requirement at Mt Eden. He said the prison had just two guards per wing.
"If somebody goes sick they don't replace them so you're left with one. So you've got one officer to 45 prisoners ... and sometimes even worse. We've walked around that prison and there's been one officer to 90 prisoners," he said.
Serco profit dropping
Earlier this month, British-based Serco slashed its profit forecasts by $40 million after writing off $3 billion worth of losses on some of its contracts.
At the time, Serco New Zealand said while the company had challenges, its business here was still sound.
However, the Public Service Association fears global cost cutting initiatives being carried out by Serco in response to its financial strife could put even more pressure on staffing levels at Mt Eden.
Serco did not respond to questions about the unions' staffing concerns or the high number of assaults at Mt Eden.
In a statement, a Serco spokeswoman said its priority was to "operate a secure, safe, decent, responsible and efficient prison."
She said Corrections rated Mt Eden Corrections Facility as "exceptional" in its league table and it was the only prison to hold that top ranking.
"We operate under rigourous reporting requirements and make sure we report every incident, no matter how minor," she added.
Mr Hanlon described the league table as a joke, saying public prisons could move up and down the chart from a single incident but Serco did not appear to have the same criteria.
"I know that when they hit the top last time, during their reporting period they had a prisoner found with his throat slit," he said.
No warnings issued
Corrections Minister Sam Lotu-Iiga declined to comment saying it was an operational matter for his department.
Labour Party Corrections spokeswoman Jacinda Ardern said she had visited Mt Eden and shared the unions' concerns.
She said the prison's privatisation had lessened the Government's accountability for what happened there.
"There's no requirement now for the minister to respond to poor performance by Serco. If we see something like a high number of incidents, he'll simply refer it to Serco as an operational matter, now that's just not good enough," she said.
The Department of Corrections said it had not issued any warnings to Serco in relation to the number of assaults at Mt Eden.
"Serco have [sic] generally been rewarded for the low rate of serious assaults at Mt Eden Corrections Facility under the contract's Key Performance Indicator Framework," a spokeswoman said.
Serco is building its second prison in Wiri in Auckland under a public private partnership agreement with the Government and the facility is due open early next year.