New Zealand officials have admitted making a mistake by allowing the man accused of killing 51 people at two Christchurch mosques to send a letter from his prison cell.

The six-page, handwritten note was posted on 4chan, a website notorious for being a place where white supremacists can post freely. 

It comes at a sensitive time, as other alleged killers from El Paso to Norway have cited the Christchurch suspect, Brenton Tarrant, as an inspiration.

The letter appears to have been written in pencil on a small notepad and is addressed to “Alan” in Russia.

Much of it appears to be relatively innocuous, discussing a one-month trip Tarrant says he took to Russia in 2015, but the letter also warns that a “great conflict” is coming and uses language that could be construed as a call to arms.

New Zealand Corrections Minister Kelvin Davis said he did not believe the prison system should have allowed Tarrant to send the letter.

“I have made myself clear that this cannot happen again,” he said.

But Davis also said that all New Zealand prisoners have rights that include the ability to send and receive mail. He said the prison system can withhold correspondence and held some other letters Tarrant had attempted to send or receive.

In the letter, dated July 4, the suspect thanks “Alan” for postage stamps he apparently sent, saying they are the only two pieces of colour in an otherwise grey cell and that he will have to hide them from the guards.

Opposition spokesman David Bennett has called for immediate answers about how an inflammatory letter could be sent from inside a maximum security prison.

“This man is accused of carrying out one of the most heinous crimes in New Zealand history,” Bennett said. “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 corrections department, which oversees prisons, said the law only allows a jail director to withhold an inmate’s mail in a “very limited” range of circumstances.

“On review, we acknowledge that this letter should have been withheld,” the department said in a statement. “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.”

Before the March 15 shootings, the suspect, a 28-year-old Australian white supremacist, posted a 74-page manifesto on the website 8chan, in which he outlined his racist views.

8chan, seen as a more radical offshoot of 4chan, was effectively knocked offline this month after two companies cut off vital technical services in response to claims that the gunman who killed 22 people at a Walmart in El Paso, Texas, posted a racist anti-Latino text on the site just before the August 3 killings.

Like the Texas gunman, a Norwegian man suspected of killing his stepsister and then storming an Oslo mosque with guns this month is also believed to have found inspiration in the Christchurch attack.

New Zealand prime minister Jacinda Ardern has vowed never to say Tarrant’s name to deny him the publicity she says he craves, making his letter even more of an embarrassment for the government.