But Davis said the "call to arms" was a matter of interpretation.

The normal process was for Corrections to deal with mail that detailed overt actions.

New Zealand had never dealt with a prisoner like this before, Davis said.

"I've asked the question, are our laws as they stand actually fit for purpose?"

He had asked for advice on changes they may need to make, Davis said.

"The safety of the public of New Zealand and internationally is our number one priority."

New Zealanders would be surprised that prisoners were entitled to send and receive mail, but there were limited circumstances where it could be withheld, Davis said.

"Corrections has put a temporary stop on this prisoner's mail until we... receive absolute assurance that it upholds the safety of the public."

Corrections chief executive Christine Stevenson was very competent and should not resign, Davis said. She had apologised for Corrections' failing over the letter.

Corrections boss apologises

The accused gunman would be blocked from sending or receiving mail pending a review, Corrections' chief executive Christine Stevenson said last night.

Prime Minister Jacinda Ardern said the alleged Christchurch gunman should not have been able to send a letter expressing what she called his "hateful views" from behind bars.

Stevenson apologised in a statement last night.

"I would like to apologise for the distress that this has caused to those impacted by the tragic events of 15 March," she said.

"We have taken swift action to ensure that our processes are as effective as we need them to be. 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."

The accused gunman had been in custody for five months, Stevenson said.

"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."