Keeping the Christchurch mosque terrorist behind bars is expected to cost the taxpayer millions of dollars over the next two years, Cabinet papers show.
Gunman Brenton Tarrant is currently awaiting sentencing in Paremoremo Prison for killing 51 people when he opened fire at two Christchurch mosques in March last year.
The Cabinet papers show $1.9 million has been approved to keep him behind bars for this fiscal year, with a further $1.6m for next year and 2021.
Money has also been allocated for screening prisoner mail after Newshub revealed last year Tarrant had sent a letter that ended up on an online white supremacist message board.
"The initiative provides a more intensive mail review service where Corrections will centralise the logging and review of mail from identified high-risk inmates," the papers say.
According to the papers, Tarrant was "likely to have needs and present risk at a level of severity beyond anyone that the Department of Corrections has managed before".
The documents recommended Cabinet approve $3.6m in operating funding to manage him.
After pleading not guilty to all charges in June 2019, Tarrant pleaded guilty to 51 charges of murder, 40 charges of attempted murder, and a charge of engaging in a terrorist act in March.
Last month, Newshub revealed Tarrant flew a drone over the Deans Ave Mosque in January - just 10 weeks before he committed the attack.
Documents released under the Official Information Act showed he launched it out of Hagley Park, just across the road from the mosque.
Newshub also earlier revealed Tarrant was believed to have prayed at the same mosque in the lead-up to the attack.
"If you look at the way he behaved in here; he knew the place like his house," Imam Gamal Fouda said.