The Department of Corrections chief executive has apologised to Christchurch mosque attack victims after the accused gunman managed to send a letter from prison.
The letter ended up being posted to the far-right message board 4chan.
Christine Stevenson told 1 NEWS the letter should not have been able to be sent and the accused will not be able to receive any mail effective immediately.
"I acknowledge that this letter should not have been able to be sent," Ms Stevenson said. "With immediate effect, this prisoner will 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."
Australian man Brenton Tarrant is being held in Auckland Prison at Paremoremo. He's accused of murdering 51 worshippers, attempting to murder 40 others and one charge under the Terrorism Suppression Act following a shooting rampage at two mosques in Christchurch on March 15.
He has been in custody for five months, and Ms Stevenson said he had been "safely and securely managed by highly professional staff, with full regard to the need to uphold the law and manage the unprecedented risk that he presents to the safety of the community, our staff, other prisoners and himself".
"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."
Under the Corrections Act 2004, sending mail is a legislatively required minimum entitlement, however mail can be withheld in a very limited number of circumstances, and Corrections have withheld some of his mail where concerns have been identified.
But Corrections Minister Kelvin Davis is now questioning whether current laws are fit for purpose and is seeking advice on what changes need to be made.
"I have made myself clear that this can not happen again," Mr Davis said in a statement yesterday evening.
"We have never had to manage a prisoner like this before – and I have asked questions around whether our laws are now fit for purpose and asked for advice on what changes we may now need to make."