The Corrections Department is investigating cell checks by prison guards following the discovery of a pocketknife hidden in the cell of maximum security inmate Graeme Burton.
Burton was jailed for killing Karl Kuchenbecker while on parole in 2007, and tried to kill a fellow inmate by stabbing him with a sharpened steel rod the following year.
He previously served 14 years in prison for murder over a fatal stabbing outside a Wellington nightclub in 1992.
The department says a search of his cell several weeks ago at Auckland's Paremoremo Prison uncovered the knife inside a fan.
Correction's Northern Regional Manager, Warren Cummins, says daily cell checks are part of its management plan for Burton, and staff are required to sign off each one.
However he says an initial review was conducted after the discovery of the knife, leading to concerns about whether the plan was being followed.
Mr Cummins says the staff members have received letters telling them an employment investigation is underway.
Burton has been charged with possessing an unauthorised item.
The Labour Party's law and order spokesperson, Clayton Cosgrove, says prisoner entitlements are minimal at Auckland prison at Paremoremo, and security should be tight.
He says the Corrections Minister, Judith Collins, needs to explain how a prisoner with such a violent record can gain access to weapons.