Prisoner attacks on other inmates and guards have nearly doubled in five years, a Corrections Department report has revealed.
Forty-eight inmates were bashed or sexually assaulted by other prisoners in the 2010/11 year, compared to 27 such incidents in the 2006/07 period.
Attacks on guards went from six to 11 during the same times.
But a Corrections spokesperson says our prisons are safe and violence has decreased dramatically since the 1990s.
Canterbury University criminology professor Greg Newbold has a different view. He says the rise in attacks is the result of increasing cramming in jails.
"When you have an increasing prison population and a set number of facilities, you're going to get overcrowding and more tensions," he said.
"Inmates get bored, they get frustrated and they take out their frustrations on the people closest to them."
A rise in double-bunking, increased lock-up hours and reduced rehabilitation programmes can all add to tensions, he said.
Twice as many prisoners (12) died of unnatural causes in 2010/11 as in the previous year.
Corrections is waiting for coroners' reports on causes of death.
Generally, deaths in jail are most commonly by suicide, but can also result from accidents, drug overdoses and killings.
But Corrections says prisons aren't becoming increasingly violent.
Assistant general manager operations Leanne Field said serious assaults are generally random and unexpected, and therefore difficult to predict and prevent.
Field said assaults have dropped dramatically since the 1990s. The rise in current year figures over five years ago was a statistical blip.
Double-bunking wasn't to blame for any rise in tensions, she said.
"There's no over-crowding in our prisons in New Zealand. We carefully undertook a programme of double-bunking and often there are benefits in that. It can give people a bit of company, which is better for them than solitude."
The prisoner assault rate was 0.55 per 100 inmates during the past year, compared to 0.35 per 100 five years ago. That was down from 1.66 per 100 in 1997/98.
Corrections defines a serious assault as an act of violence including one leading to overnight hospitalisation, sexual contact or injuries requiring ongoing treatment.
Corrections introduced stab-resistant vests and spit hoods for staff in 2009.
Prison guards have also been trained in de-escalating volatile incidents and have trialled pepper spray for defence.
Guards thwarted 200 suicide attempts by inmates over the past five years.
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