A young man who expressed admiration for the Christchurch mosque killings will be held in prison beyond the date of his previously expected release.

Frank Finch, 22, had "applauded the actions" of the alleged shooter in a letter days after the killings to his grandfather Rod Finch.

Rod Finch told the Herald today he found the letter disturbing. It talked of New Zealand being a white country, that immigration was destroying New Zealand and that immigration should be halted.

Rod Finch wants his grandson Frank Finch, who has said he wants to kill, to remain in institutional care. Photo / Facebook

He had sought the help of prisoner rehabilitation group, the Transforming Justice Foundation, to obtain support for his grandson.

Frank Finch had disclosed to Scott Guthrie, of the foundation, that when next out of prison he intended to kill so he would be put back in prison long term. Further, he disclosed he was associated with a white supremacy gang.

Frank has been having trouble with the law off and on since he was a child. He also has problems with mental health, drugs and alcohol.

He is now in Christchurch Men's Prison on remand on charges of breaching court release conditions, unlawfully entering a vehicle and theft.

In December he was a passenger in a stolen car which was involved in a police pursuit. Two others in the car died after it crashed.

He was to be sentenced tomorrow and it was expected he would be released because of the time already spent in prison on remand.

But the Department of Corrections said he had been assessed by a prison reintegration service provider.

"He has refused to engage with them and made further threats about carrying out offences. These have been referred to police.

"At Friday's court appearance we will be seeking an adjournment to enable the offender to remain in prison while a safe plan for his release can be made."

He had been referred to a supported-accommodation provider for after his release; the provider declined the referral as "the offender has previously burgled them and other residents while residing there".

Rod Finch said of his grandson: "Drug abuse is where his problem lies."

It had progressed from cannabis, to P, and he had been diagnosed with schizophrenia while in the Mason Clinic, Auckland's secure forensic psychiatric hospital.

"When he was first released from Hillmorton [forensic mental health service in Christchurch] after some time in the Mason Clinic he was in a good head space."

He believes it would be best for the public, and for Frank, if he spent the rest of his life in an institution.

"The public have been traumatised enough with his behaviour."