Wellington: New Zealand officials admitted on Wednesday that they made a mistake by allowing the man accused of killing 51 people at two Christchurch mosques to send a hand-written letter from his prison cell.

The six-page letter from Brenton Tarrant was posted this week on the website 4chan, which has become notorious as a place for white supremacists to post their views. And it comes at a sensitive time, with other alleged killers from El Paso to Norway citing Tarrant as an inspiration.

The Australian accused of committing the Christchurch terror attack has written to an alt-right supporter overseas from his prison cell. Credit:David White/Stuff

The letter appears to be 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.

Corrections Minister Kelvin Davis said in a statement that he didn't believe the prison system should have allowed Tarrant to send the letter.