Dean Joseph Shepherd attacked Bell on December 10, 2007 at a maximum security block in Paremoremo Prison.
Shepherd, 48, pleaded guilty in Auckland District Court yesterday to wounding with intent to cause grievous bodily harm, the New Zealand Herald reported.
Shepherd was serving a minimum non-parole period of 17-1/2 years for murdering his Thai-born Darunee Aphiromlerk landlady in 2004.
The court was told after the stabbing, Shepherd jumped on top of Bell "in a frenzied rage" and hit him on the back of the head with the metal instruments and his fists as he attempted to get to Bell's eyes.
Prison guards had to forcefully pull Shepherd away to stop the attack.
When police asked Shepherd for an explanation of why he did it he declined to answer.
Prosecutors have indicated they will seek a sentence of preventive detention for Shepherd.
However, his lawyer, Shane Cassidy, argued since Shepherd was already serving a life sentence that would mean the preventive detention option would be "relatively defunct".
Shepherd was remanded in custody until today when there will be legal argument on whether preventive detention applied to a life prisoner. He will be sentenced on September 11.
Bell is serving a 30-year sentence for killing three people at the Mt Wellington-Panmure Returned Services Association in 2001.
