New Zealand officials have admitted they made a big mistake by allowing the man accused of killing 51 people at two Christchurch mosques to send a hand-written letter from his prison cell, reports Australian Broadcasting Corporation citing an international news agency.

The Australian wrote a six-page letter from his prison which was posted this week on a website, which has become notorious as a place for white supremacists to post their views, added the report.

The letter appears to be written in pencil on a small notepad and is addressed to someone in Russia. Much of it appears to be relatively innocuous, discussing a one-month trip the gunman claims he took to Russia in 2015.