The alleged gunman in the Christchurch mosque attacks that killed 51 people has been exchanging letters with white supremacists from his maximum security jail cell at Auckland’s Paremoremo prison, the New Zealand government has confirmed.

A letter from Brenton Tarrant was sent to a supporter in early July and experts say it can be read as a “call to arms” for white supremacists worldwide. The letter was posted to the anonymous message board 4chan, which has become notorious as a place for white supremacists to post their views.

It is legal for prisoners in New Zealand to send and receive mail, which may only be intercepted by authorities in limited circumstances, but ministers have said Tarrant’s letter should never have been allowed out.

“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,” said corrections minister, Kelvin Davis, in a statement.

“I do not believe that corrections should have allowed this letter to be sent and have sought assurances from them that there will be an enhanced process from now on.”

Faisal Sayed, the general secretary of the Linwood mosque, where seven people died in March, said the community were “horrified” about the letter. “It’s truly frightening, and frankly, we’re shocked. It’s unbelievable,” said Sayed. “Shouldn’t he be subject to special circumstances in prison?”

“Again this shows the laws in New Zealand are lagging behind. I am really worried about the repercussions of this. He could be forming other groups, or inspiring other attacks. People are feeling very worried, and truly scared.”

The opposition party’s corrections spokesperson, David Bennett, said the corrections minister owed the victims of the mosque shootings “an immediate apology”.

“The rights of the victims in this case and respect for them should be put above everything else. This man is accused of carrying out one of the most heinous crimes in New Zealand history. 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.”

A spokesperson for the corrections department said it had subsequently changed its handling of Tarrant’s mail.

“On review, we acknowledge that this letter should have been withheld. We have 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,” the spokesperson said.

Australian self-declared white supremacist Tarrant is awaiting trial for 51 murders in two inner-city Christchurch mosques on 15 March. He also faces charges under New Zealand’s terrorism laws.

On Thursday, his lawyers asked for the trial to be moved from Christchurch, where the attacks took place. A hearing on the request to move the trial to Auckland, New Zealand’s largest city, will be held on 3 October, high court justice Cameron Mander said at a hearing.

Defence counsel did not respond immediately to a request for comment about why they asked for a change of venue.

Tarrant has pleaded not guilty and his trial is scheduled for May next year.