The Christchurch mosque shooter has launched a legal challenge due to go before a High Court judge tomorrow morning.

Brenton Tarrant is serving a sentence of life imprisonment without parole for murdering 51 people and attempting to murder 40 others at Christchurch's Al Noor Mosque and Linwood Islamic Centre on March 15, 2019. He was also convicted under terrorism laws.

Tarrant has sought a judicial review, which is due to be heard by Justice Geoffrey Venning in the High Court at Auckland tomorrow morning.

Court records show the hearing is in chambers, which means it will not be open to the public. Media are, however, permitted to attend. The records show Tarrant intends to represent himself.

It is unclear what Tarrant is challenging tomorrow morning but the Herald understands it has no bearing on his conviction or sentence.

A judicial review is where a judge is asked to review legal action or a decision. The judge looks at whether the way the decision was made was in accordance with the law - but the judge won't usually decide whether the decision was the "right" decision.

Judicial reviews are always heard in the High Court and about 180 judicial reviews are heard every year.

A special "prison within a prison" is guarding Tarrant at a huge cost to the taxpayer.

The facility known as the Prisoners of Extreme Risk Unit was set up four months after the mosque shootings and holds Tarrant and two others.

"Tarrant is in his own wing and there are 18 guards rostered to monitor him," a source told the Herald last month.

"The other two are in the same wing but they are all dealt with individually, it's a costly exercise."

Corrections says the unit cost $2.77 million in the year to October 31, excluding the salaries of the six staff in its management group. That compares to Corrections spending about $1.1 billion in 2020 to guard close to 10,000 prisoners across all its facilities.

Tarrant's life in jail without parole was the first time in New Zealand's history such a term had been imposed - meaning he "will never see the light of day again", as Prime Minister Jacinda Ardern put it.