If follows revelations yesterday that the accused Christchurch mosque gunman was able to send multiple letters, including one to Russia which later appeared online.
Tonight Corrections confirmed white supremacist Philip Arps had wrongly been allowed to send his own letter to a media organisation.
Arps is detained in Christchurch Men's Prison for distributing the Christchurch mosque shooting video.
Corrections chief executive Christine Stevenson said the second letter was brought to her attention late this afternoon.
"This is totally unacceptable, it should not have happened, and I apologise for any further distress this has caused," she said.
At least one of the letters made threatening comments about Prime Minister Jacinda Adern and also praised Norway's mass killer Anders Breivik, Newshub said.
"This is totally unacceptable, it should not have happened, and I apologise for any further distress this has caused," Stevenson said.
Changes to letter vetting system:
Stevenson said she did not have confidence in current processes for reviewing and assessing prisoners' mail, and had called for an immediate review.
"I have just briefed the Minister and given him an assurance that the following things will happen immediately:
"The mail of prisoners who have been identified with extremist ideologies and/or registered victims will be immediately centralised pending a full review carried out by an external party (to be determined).
"It will remain this way until the review has concluded and I am confident the new process in place will prevent this from ever happening again."
Stevenson said the occurrences had also highlighted the need to look at the current legislation, how it was being applied and whether it was fit for purpose.
"I want to reiterate my unreserved apology for the distress that this has caused to those impacted by the tragic events of 15 March.
"We take our responsibility for safety very seriously, and on this occasion we have not met our own high standards."
Corrections Minister Kelvin Davis said it was wrong that a letter like this could be sent.
"There are clearly deficiencies in the current management of some high-concern prisoners' mail.
"I've met with the Corrections chief executive this afternoon and she will be immediately putting in place changes to the system by centralising the review of these prisoners' mail into a single specialist team," he said.
"The letter is further proof we need to ensure our current laws are fit for purpose. I will discuss the issue with Cabinet on Monday."
Accused mosque gunman's prison letters:
Last night it came to light that during the accused gunman's five months in custody, he had sent seven letters.
Two of the letters went to family, another to an admirer overseas and four to recipients yet to be identified.
A further two letters he wrote have been withheld.
The alleged killer has pleaded not guilty to murdering 51 men, women and children at two mosques and will go on trial next year.
In one of his letters the accused wrote about his political and social views but said he could not go into great detail about his regrets or feelings "as the guards will confiscate my letter if I do [to use as evidence]".
Part of the letter included what has been described as a "call to arms".
After the letter was posted on the internet this week, Stevenson said the accused would be blocked from sending or receiving mail pending a review.
"This letter should not have been able to be sent, and this prisoner has immediately been prevented from sending or receiving 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," she said.
"The strengthened process which we are going to implement is unprecedented, and demonstrates the seriousness with which we are taking this, and our commitment to ensuring that we get this right."
Stevenson said the revised process will include all of his correspondence being reviewed by a Multi Disciplinary Team, partner agencies with specialist knowledge, and the Chief Custodial Officer.
She said the final decision about withholding the prisoner's correspondence will be made by herself, in accordance with section 108 of the Corrections Act.
"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," she said.
