Department of Corrections northern assistant regional manager Jeanette Burns said George Baker was being transported back to Auckland's Paremoremo Prison from hospital on July 5 and was in the back of the van with three corrections officers.
He produced an improvised weapon, made from a small piece of metal, which he had "concealed on himself internally".
Baker was handcuffed to one of the officers, but was not wearing a waist restraint, brought in after Liam's death, because his arm was "seriously injured", Ms Burns said.
"Staff acted immediately and appropriately to the situation. They restrained the prisoner and removed the weapon from him."
One officer received a minor cut to his finger and received medical treatment "as soon as was practicable".
The department could not say why Baker was in hospital, for privacy reasons, but said it was not the result of an altercation with staff or other prisoners.
No other prisoners were involved in the latest van incident.
The waist restraints, which tether a prisoner's wrists to a waist belt, are used when prisoners are being moved.
They were introduced after 17-year-old Liam was murdered by Baker in a prison van in August 2006.
In December 2006, Baker was jailed for a minimum of 18 years in prison for the murder.
