Northland police are investigating another assault at Ngawha Prison - the second such incident at the prison this year.
A prisoner has been flown to hospital with serious eye and head injuries after an attack at the prison on Sunday.
Police say the 56-year-old man was attacked by another prisoner, who is now in isolation.
In March, a prisoner was critically injured when he was kicked and stomped on by fellow inmates.
The Corrections Department says Ngawha Prison was temporarily locked down after Sunday's incident, and a lockdown in one section of the prison on Monday was routine and unrelated to the assault.
Meanwhile, sections of Auckland Prison at Paremoremo prison are also locked down while staff carry out a search of the east division.
Two inmates prison were hospitalised last week after a stabbing incident involving four prisoners.