Staff at Christchurch men's prison are scouring cells, in search of a weapon that could've been used to stab an inmate.

The Corrections Association says three men launched a pack attack on a lone prisoner yesterday, who remains in Christchurch Hospital with serious injuries.

Spokesperson Beven Hanlon says staff have one unit on lock-down, and are trying to piece together what happened.

"They're not sure about the weapon as yet. They believe he was stabbed because of the amount of the blood there.

"Until they find what it is, they're not sure what's been used."

Hanlon says days earlier an inmate at Rimutaka Prison tried to stab a staff member with a 30 centimetre piece of steel.

"Staff are literally physically rolling around with the prisoner that's kind of stabbed him and they're trying to take control of him and it was quite a struggle.

"It was essentially a life or death struggle for those staff."

Hanlon says prison staff should have access to tasers, protective vests, and pepper spray.