Corrections says a letter sent out of prison by the alleged Christchurch shooter should have been "withheld".
On Tuesday a post appeared on /pol/, a 4chan thread infamous for graphic content and controversial comments, which read: "A couple of months ago I sent a letter to Brenton Tarrant and today I finally got a response."
The user, who claims to live in Russia, posted a photo of an envelope with a stamp that says 'Auckland Prison'. The name Brenton Tarrant is written above the return address, which is that of the Paremoremo prison.
Six more photos were published, one of each page of the letter, which is dated July 4, 2019.
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Tarrant is currently awaiting trial in solitary confinement in Auckland Prison for allegedly murdering 51 people during attacks on two mosques in Christchurch on March 15.
Much of his letter appears to be innocuous. However, it ends with what is effectively a call to action for like-minded people, which Newshub has decided not to share. This is then signed off by the alleged gunman.
Ben Elley, an expert in online extremism, says the final line was enough to provoke a reaction among the far-right.
"I think there certainly are people out there in those communities who will read it as a call to arms," he told Newshub.
"Whether that's a call to commit other acts of terror or not isn't clear - people will interpret it in their own way."
Corrections Minister Kelvin Davis says he is angry at his department for letting this letter get out.
Corrections issued a statement saying "we acknowledge that this letter should have been withheld".
"Corrections is legislatively required to manage prisoners in accordance with the provisions set out in the Corrections Act 2004 and our international obligations for the treatment of all prisoners," a spokesperson told Newshub.
"Section 69 of the Act states that every prisoner has legislatively required minimum entitlements. One of these minimum entitlements is to send and receive mail. In accordance with section 108 of the Act, a Prison Director can only withhold a prisoner's mail in a very limited range of circumstances. Some letters have been withheld.
"We have made changes to the management of this prisoner's mail to ensure that our robust processes are as effective as we need them to be."
Arthur Taylor, who spent a long time in Auckland Prison, told Newshub Tarrant has a "statutory right to send and receive as much mail as he wants" but inmates have to pay for anything more than three letters a week.
He says Corrections staff "definitely don't read everything" that comes in and out of prison.
While Tarrant has the right to send and receive correspondence, it would be a different story if he tried to send a letter containing ideas about how to carry out an attack, or if it featured "blatant" white supremacist slogans.
Mail to and from inmates may be withheld if it endangers anyone's safety or promotes the commission of an offence, according to the Prison Operations Manual.
In late March, users on the now-offline message board 8chan shared instructions for how to write to Tarrant, listing the prison address as well as tips to get letters past Corrections staff. Some mentioned sending him money to use in prison as well.