Brenton Tarrant, an Australian former gym instructor, admitted carrying out New Zealand’s deadliest modern-day mass shooting before being sentenced to life in jail in August 2020.

Now, the convicted killer argues that his “torturous and inhumane” detention conditions during his trial made him incapable of making rational decisions when he pleaded guilty, according to a court synopsis of the case.

If the Court of Appeal in Wellington upholds his conviction, it would hold a separate hearing later in the year to consider an appeal against his sentence.

His penalty of life imprisonment without parole was the stiffest in New Zealand history.

Tarrant filed his appeal out of time, so would need the court’s leave for it to proceed.

Armed with an arsenal of semi-automatic weapons, Tarrant attacked worshippers at two mosques in Christchurch on March 15, 2019.

He published an online manifesto before the attacks and then livestreamed the killings for 17 minutes.

His victims were all Muslim and included children, women and the elderly.

After Tarrant’s livestream, Facebook said it removed 1.5 million videos that proliferated within the first 24 hours showing the harrowing viral footage.

Then-prime minister Jacinda Ardern quickly moved to tighten gun laws in the aftermath and put pressure on social media giants to curb online extremism.

In 2021, Tarrant’s former lawyer, Tony Ellis, said his client had believed “the simplest way out was to plead guilty”, arguing the plea was made under duress.

The names of Tarrant’s current lawyers have been suppressed by the court.