Police said the young man charged over Friday's twin mosque attacks in the New Zealand city of Christchurch is believed to have acted alone.

Australian Brenton Tarrant, 28, a self-described white supremacist, live-streamed the attack on Facebook.

Three others arrested afterwards are not believed to have been involved, Police Commissioner Mike Bush said, but added that he could not be conclusive.

Fifty people were killed and 50 injured in what is New Zealand';;;s worst attack.

Some 34 people remain in hospital receiving treatment for injuries ranging from severe gunshot wounds to "relatively superficial soft tissue injuries", authorities said.

A four-year-old girl remains in a critical condition.

Commissioner Bush said authorities were working as fast as they could to finish formally identifying the victims of the attacks at the Al Noor and Linwood mosques.

He added that it was a sensitive process and that he was "aware of the cultural and religious needs".