The gunman who killed 51 worshippers at two mosques in Christchurch, New Zealand is considering appealing against his convictions and prison sentence, his lawyer has said.

Australian Brenton Tarrant was subject to inhumane and degrading treatment in jail, prompting him to plead guilty under duress, lawyer Tony Ellis wrote in a memo to New Zealand’s chief coroner.

Tarrant, a white supremacist, livestreamed the 2019 attacks on Facebook. It was the worst atrocity in New Zealand’s modern history and prompted lawmakers to swiftly ban the deadliest types of semiautomatic weapons.

Last year before his trial was due to begin, Tarrant pleaded guilty to all charges, including 51 counts of murder, 40 counts of attempted murder and one count of terrorism. He was sentenced to life imprisonment without the possibility of parole, the toughest sentence available.

The lawyer’s memo was not immediately made publicly available on Monday. Mr Ellis said the gunman had authorised him to discuss the case with only two local media outlets, RNZ and Stuff.

The coroner’s office did not immediately release a copy of the memo, referring requests back to Mr Ellis. But the coroner’s office did not dispute the memo’s existence or the characterisation of it by the media outlets.

RNZ reported Mr Ellis had advised his client to appeal his sentence and convictions on the basis that his rights had been breached, and that Tarrant was considering such an appeal.

Mr Ellis said the gunman was held in solitary confinement for much of the time while awaiting his trial and lacked proper access to lawyers, information and documentation about his case.

The gunman told his lawyer his treatment in jail affected his “will to carry on” and he had decided the simplest way out was to plead guilty, RNZ reported.

The memo came after chief coroner Deborah Marshall’s office last month opened an inquiry into the deaths of the victims, the latest in a series of probes into the attacks.

Rosemary Omar, whose 24-year-old son Tariq was gunned down at Al Noor mosque, told RNZ the gunman’s claims were nothing more than him seeking further attention.