In March last year, Warren shot at Tauranga Sergeant Logan Marsh and Constables Regan Mauheni, Damian White and Andrew Finn as they entered his rural property near Kawerau, intending to remove cannabis plants.
One of the officers was hit in the head, the bullet splitting his skull.
Warren then barricaded himself in the Onepu Springs Rd home during a 22-hour stand-off with police. He admitted firing shots but said it was in self-defence.
Warren represented himself during the trial, telling the court he feared he was fighting for his life when the armed officers entered the house.
Inspector Warwick Morehu who knew Warren's family talked to Warren in an effort to encourage him to end the siege and give himself up.