'This cannot happen again': Prime Minister and Corrections Minister respond to alleged Christchurch shooter Brenton Tarrant's letter

The Corrections Minister says a letter sent by the alleged Christchurch shooter should never have been allowed to be sent.

Brenton Tarrant, who is accused of murdering 51 people during attacks on two Christchurch mosques earlier this year, was allowed to send the letter to someone in Russia, Corrections confirmed Wednesday.

The recipient posted a photo of the letter to the 4chan forum on Tuesday, saying "A couple of months ago I sent a letter to Brenton Tarrant and today I finally got a response".

But Corrections Minister Kelvin Davis says his department made a mistake in allowing the letter to be posted.

"I do not believe that Corrections should have allowed this letter to be sent, and I have sought assurances from them that there will be an enhanced process from now on," Davis says.

"I have made myself clear that this cannot happen again."

Speaking from Tuvalu, Prime Minister Jacinda Ardern echoed Davis' words.

"This particularly bit of communications just should not have happened," she told media.

The letter is dated July 4 and the return address is that of Paremoremo prison. At the end, the letter contains what appears to be a call to action to his supporters.

Earlier today, the Department of Corrections said that following the incident it had 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".

But National's Corrections spokesperson David Bennett says changes after the fact are not good enough. 

"This man is accused of carrying out one of the most heinous crimes in New Zealand history," says Bennett.

"New Zealanders will be horrified that Corrections allowed him to send a letter which includes a call to action and has subsequently been posted online.

"The minister is now refusing to comment – he owes the victims of this crime an immediate apology."

Bennett says that the rights of the victims should be put above anything else in this case.