The Department of Corrections has conceded that the man accused of the Christchurch terror attack should not have been able to send a letter to an admirer from his cell at Auckland Prison.

And the alleged murderer - who will stand trial for gunning down 51 men, women and children on March 15 - is now blocked from sending or receiving mail pending a review.

So what are the rules for inmates when it comes to correspondence in and out of prison?

Inmates send and receive thousands of letters each year and Corrections supposedly has strict rules about what they can send, and to who.

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It was revealed last night that the accused sent a series of letters in recent months - to his family, an overseas admirer and other yet-to-be identified recipients.

The admirer posted the handwritten letter - in which the accused talks about his political and social views and a "call to arms" - online.

Corrections later apologised, saying he would not be able to send or receive any mail "until we have absolute assurance that the process in place for screening and assessing his correspondence upholds the safety of the public, both in New Zealand and internationally".

According to Corrections rules, inmates are able to send and receive letters to and from "friends and family outside of prison".

"Corrections provides writing paper and envelopes, and pays for standard postage for up to three letters per week," the rules state.

"If the person wishes to send a fast post letter or a parcel, they must pay for this themselves."

All letters are checked by staff for contraband and mail may also be checked by drug dogs - often resulting in a delay in prisoners receiving them.

The admirer who wrote to the accused stated they had sent him the stamps to reply.

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The Herald has sought comment from Corrections as to whether that is allowed.

Most other items are not allowed to be posted to prison and are considered contraband.

"People can also send a cheque or set up an electronic bank transfer to be deposited into the person's prison trust account so that they can buy phone cards and other approved grocery items, such as toiletries and snack foods," the rules state.

"To send a prisoner other items, like clothing, books or CDs check with the prison directly before sending – each prison has its own processes for incoming property."

The Herald has also sought further comment from Corrections on how the alleged gunman's inward and outbound correspondence was being monitored.

Last night Corrections chief executive Christine Stevenson said the letter to the admirer, based in Russia, "should not have been able to be sent".

She apologised for any distress caused to those impacted by the tragic events of 15 March.

"We have taken swift action to ensure that our processes are as effective as we need them to be," she said.

"It is a fine balance to uphold our lawful obligations and mitigate all potential risks posed by the prisoner, however we are absolutely committed to ensuring that he has no opportunity to cause harm or distress, either directly or indirectly."

Stevenson said the agency was "legislatively required" to manage prisoners in accordance with the Corrections Act 2004.

"There were international obligations for the treatment of all prisoners.

"Sending mail is a legislatively required minimum entitlement under the Act.

"Mail can be withheld in a very limited number of circumstances, and we have withheld some of his mail where concerns have been identified.

"We have never managed a prisoner like this before and the risk he presents is unparalleled.

"Our overriding priority is the safety of the public, and we will continue to work with our partner agencies to ensure that we have the right skills, capability and experience to continually assess any threat that this prisoner, or his correspondence poses.

"In addition, we are working to provide our minister with advice about what legislative changes could be made to further strengthen our management of this prisoner."