Convicted killer Dean Shepherd has admitted attacking triple-murderer William Bell at Auckland Prison at Paremoremo.
Bell was stabbed in the eye at a maximum security unit at the prison in 2007.
He is serving one of the longest sentences handed down, for the murders of three people at the Mt Wellington RSA in 2001.
Dean Shepherd has pleaded guilty to a charge of wounding with intent to cause grievous bodily harm to Bell.
Shepherd was convicted of murdering a woman and dumping her body in the Waikato River, and is serving a 17 year jail term.
The plea was entered in the Auckland District Court on Monday. A sentencing date is still to be set.