The 6-page letter was posted online on the controversial image board 4chan, warning about "a great conflict" to be coming.
The Australian white supremacist accused of terror attacks at two mosques in New Zealand that killed 51 people in March, has written several letters from jail expressing his racist political and social views, prompting Prime Minister Jacinda Ardern to say that should never have been allowed.
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"I think every New Zealander would have an expectation that this individual should not be able to share his hateful message from behind bars," Ardern was quoted telling reporters on Wednesday in Tuvalu, where she is attending the Pacific Islands Forum.
"Obviously, this is an offender who has a very specific goal in mind, in terms of sharing his propaganda, so we should have been prepared for that."
Corrections Minister Kelvin Davis said on Thursday 29-year-old Brenton Tarrant has sent nine letters from prison so far, two to his mother, five to others, while two were withheld.
New Zealand site Newshub said the last two lines of the letter posted on 4chan could be read as a call to arms but blanked out what it said.
A lone gunman armed with semi-automatic weapons attacked Muslims attending Friday prayers in Christchurch on New Zealand's South Island on March 15, killing 51 people in the country's worst peace-time mass shooting. The attacker broadcast the shooting live on Facebook.
Tarrant has pleaded not guilty to all 92 charges against him.
He did not appear via video-link as he had at previous court hearings. Mander said in a court minute Tarrant was excused from appearing because the hearing was mainly about legal arguments.
The man is in custody at a high- security prison in Auckland, about a 90-minute flight from Christchurch.
The court hearing was attended by around 40 members of the public, including survivors of the attack, although there were fewer than at previous court appearances.
Prosecution lawyers said they were making inquiries about the possibility of delaying the trial, which has been set down for May 4, by three or four weeks.
Lawyers of the man accused have asked for the trial to be moved from the southern city of Christchurch, where the attacks took place, a judge said Thursday.
A hearing on the request to move the trial to Auckland, New Zealand's largest city, will be held on Oct. 3 and Tarrant would be held in custody until then, High Court Justice Cameron Mander said at a hearing.
The trial date set down in June has been criticized for coinciding with the Islamic holy month of Ramadan when Muslims will be fasting.