Private prison operator Serco has confirmed an inmate at Mt Eden prison attempted suicide and was taken to hospital early on Saturday morning.
Earlier this year, the Auditor-General expressed concern at the high rate of unnatural deaths amongst prisoners.
Twelve inmates committed suicide in 2011, an increase of 147% on the previous year.
Serco has been running Mt Eden prison for two years and Saturday's incident is the second case of serious self harm during that time.
The operator says it introduced the Care Support and Integration Unit when it took over the prison, designed to identify prisoners at risk of self harming.
It's not known whether the inmate prisoner involved in the latest incident was assigned to that unit.
Corrections Minister Anne Tolley says the prison has a very good record in looking after at-risk inmates despite the fact many are determined to take their own lives.