The Corrections Department denies it has a systemic problem with bomb threats in prisons, despite nearly three dozen hoaxes being reported in the past five years.
Details about the threats have been released to RNZ under the Official Information Act.
Since 2012, 32 bomb threats have disrupted New Zealand jails - the majority at Manawatu Prison just outside Palmerston North - but all have been hoaxes.
The potential danger can involve the defence force, fire service and police being brought in to help.
Corrections chief custodial officer Neil Beales said each threat was treated very seriously and most were phoned in by inmates themselves.
"They are very often are made from inside the prison, and very often its purpose is to try and disrupt something happening for them, such as maybe a visiting justice hearing. The intent is malicious."
The hoaxes disrupt family visits and work arrangements, as sites are put in lockdown.
Corrections said it could not detect any "rhyme nor reason" why Manawatu topped the list.
Neil Beales said there were "many examples" of perpetrators being identified, but he could not specify how many have been charged for bomb threats.
"It would be unfair to tar everybody with the same brush and say... we've got a systemic issue with bomb threats, because we don't.
"It is something that we plan for in terms of our incident response, it's something that some prisoners will take advantage of on occasion because it's quite easy to do."
The main union representing prison guards, the Corrections Association, said bomb threats were incredibly stressful for prison staff regardless of whether they were a hoax or not.
"That's always in the back of your mind - is this the one? Is this the time there is a device somewhere sitting in that prison, and if so when's it going to detonate?" union president Alan Whitley said.
Hoaxers 'nigh on impossible' to identify
A former inmate at Manawatu Prison, where most of the hoaxes occur, said it's "pathetically easy" to phone a threat in.
"It's just become copycat crap. It's so disruptive, it throws all the screws [guards] in turmoil. They've got to run around, search the entire prison, which means... searching for bullshit which doesn't exist and they know it but they have to follow procedure."
The violent offender, who spent 19 years behind bars, said it was "extremely hard... nigh on impossible" to identify perpetrators who phone in the hoaxes.
The union said prisons could be doing more with track and trace technology to crack down on hoax calls.
But Corrections' Neil Beales said the department can track phone calls and find out their origin.
He said restrictions can also be placed on prisoners' phone use.
"It's not a case of 'we've just had another threat so we'll just deal with it and then move on'. They are disruptive, they are dangerous... it's a distraction to the programmes, it's a delay, it's inconvenient to fellow prisoners.
"Of course we take these things seriously, and we try and minimise and mitigate these events wherever we can."
Bomb threats at New Zealand prisons since 2012
- Manawatu Prison - 12
- Rimutaka Prison - 4
- Mt Eden Prison - 8
- Hawke's Bay Prison - 2
- Waikeria Prison - 1
- Whanganui Prison - 1