The man accused of carrying out the New Zealand mosque shootings 'beefed up' ahead of the massacre with an 'obsessive' fitness regime at a gym, it has been claimed.

Brenton Tarrant is said to have been deadlifting weights of up to 440 lbs (200kg) at a gym opposite an Islamic school and childcare centre in Christchurch.

Dr Mohammad Alayan, the owner of the centre situated just across the road from where Tarrant trained, was critically injured in the shooting at the Al Noor Mosque on Friday.

His son, Atta Elayyan, was shot and killed by the alleged gunman, who had compulsively attended the Anytime Fitness gym where the treadmills overlooked the centre his father owned.

A man who attended the same gym as Tarrant said he initially believed the 28-year-old was a powerlifter, and claimed he had 'beefed up really quickly' in the lead up to the attack.

Brenton Tarrant (pictured), the man accused of carrying out the New Zealand mosque shooting, 'beefed up' ahead of the massacre with an 'obsessive' fitness regime at a gym, it has been claimed

Tarrant grew up in the north-eastern NSW town of Grafton, and his childhood friends still living there said he was a loner for most of his life. He played for the Grafton Ghosts under 15 rugby league team in 2005 (Tarrant is pictured top left)

A fellow gym-goer claimed Tarrant had 'beefed up really quickly' at Anytime Fitness (pictured) in the lead-up to the attack

'[Tarrant] did mostly strength training, like deadlifts, like powerlifting – the massive ones. He's a strong man,' said the gym-goer, named as Adam.

'He was probably doing 140 kilos on the bench, which is quite a lot, and 200kg deadlifts.'

Tarrant did not say much at the gym, he said, except to ask him how he was doing, and to talk about his own travels.

Dr Mohammad Alayan (pictured left) was critically injured in the mosque attack, while his son Atta Elayyan (pictured right) was killed

'He'd just tell me about all these holidays he'd been on, Europe, France and all that, he'd ask me how my Christmas was – he was real polite,' Adam said.

Neighbours said Tarrant had become noticeably more muscular in the months leading up to the shooting at Al Noor mosque, and Adam readily agreed before suggesting he may have trained using steroids or other supplements.

'There's always the chance he's unnaturally training,' he said. 'If he's been travelling a lot it's definitely easy to get.'

Tarrant allegedly live-streamed himself walking into Al Noor mosque on Friday afternoon and callously shooting Muslims who were attending a prayer service.

He then left, and allegedly shot at members of a second mosque.

Minutes before the horrifying attack, he sent out a bizarre 73-page manifesto, which explained his hatred towards Islam and the religion's followers.

The document also revealed the attack was initially planned to take place in Dunedin, where he lived, before he decided to move it to Christchurch, where there were more Muslims.

Adam said he was 'creeped out' when he heard what Tarrant had done, and revealed he was friends with one of the victims.

'One of my mates was in [the mosque], and he got shot,' he said.

The gym Tarrant attended was opposite an Islamic school and childcare centre (Posters advertising the Islamic school and childcare centre picture)

The image shows the Anytime Fitness Tarrant attended was just across the road from the Islamic school and childcare centre

'He pretended to be dead, he didn't get killed.'

Mark Daly, a spokesperson for the Anytime Fitness gym Tarrant attended, told Daily Mail Australia they were 'horrified' by Tarrant's alleged acts.

'We are horrified and saddened by this hateful act. Our thoughts are with the victims, their families and all New Zealanders,' Mr Daly said.

The Christchurch shooting was New Zealand's first mass shooting since 1994.

Tarrant is alleged to have stormed the Al Noor mosque and later Linwood Masjid on Friday, where he shot dead 50 people and left 50 more people injured.

Twelve of 50 people injured remained in critical condition over the weekend, including one child.