Newshub has obtained details of prison guards pepper-spraying an inmate who was self-harming - a practice condemned by a human rights lawyer.
The inmate was sprayed with Cell Buster twice, but the Corrections Minister is defending their actions, saying they saved the inmate's life.
However he is reviewing regulations around when pepper spray can be used.
Cell Buster is specifically designed to incapacitate prisoners. If someone refuses to leave their cell, guards use it to fill the room and force them out.
It has been used 27 times since 2016.
"The person is in excruciating pain feeling like, from head to toe, they're on fire," explains human rights lawyer Douglas Ewen.
One of those times, it was even used on a prisoner who was self-harming. Guards wanted to move the man to another cell to search for anything he could harm himself with.
He refused, stayed in bed, covered in blankets. Guards used Cell Buster on him twice.
"Pepper spray is not gassing prisoners. Pepper spray is a very well-used tool, it's designed to make sure everyone is safe," said Corrections chief custodial officer Neil Beales.
Corrections say it was used twice because it had minimal effect.
The man then assaulted guards when they handcuffed him. After moving him, he again attempted to assault guards, at which point they used a different form of pepper spray.
Ewen is suing Corrections on behalf of Karma Cripps, an asthmatic, who was pepper-sprayed in her cell. He calls Cell Buster barbaric and wants it banned.
"There is a case to be made it is a weapon of torture and banned both in domestic law and international law," he says.
The Human Rights Commissioner agrees.
Paul Hunt told Newshub Nation Cell Buster could be a weapon of torture: "I'm deeply concerned about the use of force."
The Corrections Minister is refusing to ban its use - even in incidents of self harm.
"Without the work of the Corrections officers, that person might not be alive today. So it's not the greatest but the alternative is worse," said Kelvin Davis.
The question is whether there's a better alternative to pepper-spraying suicidal inmates.
Corrections is reviewing regulations about when Cell Buster should be used - but Newshub understands it won't look at banning it altogether.
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