The Human Rights Commission has backed a call from a UN committee for a convicted murderer to be paid compensation.
The UN Committee Against Torture has ruled inmate John Vogel had his human rights breached after being unlawfully held in solitary confinement.
In 2000, Vogel was confined to a cell for drug offending. He asked for 21 days - six days longer than legally allowed - to help him deal with his addiction.
The UN committee ruled the punishment was disproportionate given Vogel was known to be suffering from chronic depression.
Vogel's lawyer Tony Ellis lodged a complaint with the United Nations Committee in 2015 after a series of legal setbacks in New Zealand.
Dr Ellis said $10,000 to $20,000 would be a reasonable sum of compensation.
The government turned down a request for an interview, but in a statement said it would give "careful consideration" to the recommendations.
New Zealand Human Rights chief commissioner David Rutherford said it was disturbing correct processes were not followed.
"We shouldn't be having to resort to the New Zealand courts or UN treaty bodies to get things right in so far as how we treat people in prisons."
He said New Zealand was obliged under UN conventions to pay compensation and could suffer further criticism if it did not comply.
"These are conventions that New Zealand has voluntarily signed and ratified, so let's comply with them."
He said New Zealand's processes regarding prisoners needed to improve.
"Solitary confinements are very well understood to be prone to causing mental health issues, and certainly people with mental health issues shouldn't be in solitary confinement."
Vogel was sentenced in 1988 to life in prison for murder and was released a decade later, but was recalled to prison in January 2000 after further offending.
He is currently in Auckland South Corrections Facility.