Newshub has received multiple letters from Philip Arps - another white supremacist - who has been busy writing hate-filled letters.
As a result of the Arps letter, the Prime Minister's office has told Newshub changes to the system will be considered by Cabinet on Monday.
Corrections chief executive Christine Stevenson apologised and told Newshub it should not have been sent and it is "totally unacceptable".
She said the process for reviewing mail from prisoners with extremist views has since been changed and will now be centralised and handled by a specialist team.
All of Tarrant's mail will be reviewed by a multidisciplinary team, partner agencies with specialist knowledge and Corrections' chief custodial officer. Stevenson will make the final call on whether to withhold his correspondence.
Arps is the Christchurch man jailed for sharing a video of the Christchurch terror attack.
The letter of complaint to Newshub featured the unhinged ramblings of Arps - the racist who left a pig's head at a mosque in Christchurch.
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The letter is vile and violent. Newshub won't be releasing the full details of its contents.
It mentions harming Jacinda Adern, referring to her as a worthless "communist", and speaks of Arps' desire to see a former Prime Minister publicly executed.
He also praises Norway's mass killer Anders Breivik.
The Prime Minister's office has told Newhsub the Government will now consider changes to the letter vetting system that will go before Cabinet on Monday.
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The proposal is to ensure all letters from inmates with extremist views are sorted and checked in a centralised location.
There are plenty of other examples of letters that Corrections should have stopped.
Another letter threatening former inmate Arthur Taylor came from Robert Conchie Harris, a convicted double-killer and courtroom liar.
"It simply comes down to prison officers not doing their job properly," says Taylor.
After executing a couple in Whangarei in 1983, Harris gloated to his girlfriend that killing them was "just like having an ice cream".
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Taylor took a perjury case against Harris and won.
In his letter to Taylor, his researcher Mike Kalagher and his legal team in January this year, Harris states: "We will meet in the near future... I'll bring the ice cream to the party, you's can be sure of that.
"A sport I have missed dearly is hunting. Believe me, I have the instincts of a hunter and love the chase, but it's the kill that gave me a deep visceral satisfaction."
"We took it as a threat to our safety and lives. He threatened Mr Kalagher, he threatened me, he threatened Murray Gibson," says Taylor.
Police say they have investigated the case and will ensure Harris will be unable to send these threatening letters again.
The union for Corrections officers says its staff are also targets for threatening letters from prisoners but it's ultimately management who decide whether something should be released or not.
"Rules restrict what we can do. All we can do is scan a letter. If something stands out then that letter is passed to a manager to make a decision about what happens," says Corrections Association NZ president Alan Whitby.
George Baker, who murdered Liam Ashley in the back of a prison van, wrote to a female prison guard from his cell in 2010. It was sent to her home address and asked if they could be friends.
In those circumstances, a chilling request.