George Baker is an evil man and should only be let out of prison in a coffin, says the father of the teenager he killed four years ago.

Baker was given preventive detention with a minimum term of 16 years when he was sentenced in the High Court at Auckland today for attacks on a fellow inmate at Auckland Prison at Paremoremo and a guard in a prison van.

He is already serving life imprisonment, with 18 years non-parole, for Liam Ashley's murder in 2006.

He strangled and stomped the 17-year-old to death in the back of a prison van taking them to Mt Eden Remand Centre in Auckland.

In August after separate trials he was convicted of attacking a prison guard on a van on the way back to prison from Middlemore Hospital where he had been treated for a self-inflicted wrist wound and taking an 82-year-old fellow inmate hostage and threatening to kill him. On each occasion he was armed with makeshift weapons.

During today's sentencing before Justice Patricia Courtney one of Liam Ashley's friends sat behind the glass-walled dock where Baker, handcuffed to a security guard, sat.

The friend, who said his name was Chris, said Baker had kept looking at him.

Baker stood up but was held by a security guard.

"I'll take you out boy," he told Chris before the judge warned members of the public to keep quiet or face being ejected from the court.

The incident came only moments after Baker told the court he needed help in prison to cope with his violent tendencies.

Outside the court Liam's father Ian Ashley said Baker showed his true colours in front of the judge.

He said Baker was evil and should never be returned to the community and today's sentencing was closure for the family.

"He is finished with the family. He can do no more harm to us. This is closure for us. He will never ever come out of prison unless it is in a coffin.

"There is a small percentage, thank goodness, in this country exactly like him and this should send home a message to them all that life will mean life in this country."

He said Baker did not have serious mental issues.

"He is evil. You are talking about an evil person."

Today's sentence means that Baker will serve at least 16 years before he can be considered for release.

Justice Courtney said there was still a high risk of him offending again.