A convicted killer has today admitted stabbing a fellow prisoner with a home-made shank in a "frenzied attack".

Terry Jason Nahi is serving a life sentence for the murder of 21-year-old Christchurch woman Lisa Caroline Hurrell in 1998.

Today at Christchurch District Court, Nahi pleaded guilty to a brutal prison assault at Christchurch Men's Prison on May 4 this year.

Nahi, 42, armed himself with a shank and walked into an inmate's cell in the Rawhiti 2 wing about 8.15am, the Crown summary of facts says.

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"What the f*** are you doing Terry?" the victim was heard to say.

Nahi attacked the victim before the melee spilled into the corridor of the ground floor wing.

The victim was on his back when Nahi stabbed him multiple times about the head and top half of his torso.

"The victim attempted to defend himself by placing his hands up to protect his face, which had little effect," the summary says.

The "frenzied attack" lasted for approximately two minutes and only stopped when Corrections staff restrained Nahi.

While Nahi was being taken away, he shouted, "I'll get you, you f***. You'll still be here when I get back".

The victim was hospitalised with multiple serious stab wounds to his head and face, and defensive wounds to his hands.

Nahi pleaded guilty to wounding with intent to cause grievous bodily harm.

He will be sentenced on February 17.