Brenton Harrison Tarrant, 28, has been charged with murder in relation to the massacre on Friday, in which 49 people were killed and 42 injured.
"There is one charge of murder brought at the moment, it is reasonable to assume that there will be others", said the judge, whose name was not reported by the AP.
Tarrant has been identified as a suspected white supremacist, based on his social media activity.
CNN has not been able to independently confirm any information about the attackers or the alleged video.
"This attack has been an enormous shock for all New Zealanders, and I am aware that there is a real sense of fear and concern for personal safety, particularly among our Muslim communities", New Zealand police commissioner Mike Bush said.
A pair of rural New Zealand police officers dramatically arrested the suspected Christchurch gunman 36 minutes after authorities were alerted, it emerged Saturday, as the prime minister hailed their bravery.
Two other armed suspects were taken into custody Friday while police tried to determine what role, if any, they played in the cold-blooded attack that stunned New Zealand, a country so peaceful that police officers rarely carry guns. GUN LAW REFORMS Ardern said Tarrant was a licensed gun owner who allegedly used five weapons, including two semi-automatic weapons and two shotguns, which had been modified.
The floral tributes grow on Deans Avenue near the Al Noor Mosque as locals pay tribute to those who were killed March 16 in Christchurch, New Zealand. This, according to her, allowed the Australian terrorist to legally obtain the weapons in December 2017, and that he was not on any watch-lists prior to the attacks.
'I can tell you one thing right now, our gun laws will change, ' Ms Ardern told reporters.
There was a heavy police presence at the hospital where families of the more than 40 wounded had gathered.
The hospital also confirmed 39 people are still receiving treatment, with 11 of them in intensive care. Funerals were planned on Saturday for some of the victims, several of whom were born overseas.
Mourners write condolence messages in chalk along the footpaths in Christchurch.
Tarrant is believed to be the author of a 74-age manifesto posted online before the attacks occurred.
"You can not imagine how I feel", he said. "He loves New Zealand".
One man who said he was at the Al Noor mosque told media the gunman burst into the mosque as worshippers were kneeling for prayers.
Fareed Khan, a consultant and community activist, said he wants national leadership to counter the spread of hate speech targetting minority groups, including Muslims.
Political across Asia and the Middle East voiced concern over the targeting of Muslims.
I felt the need to come out and stand in solidarity with people who were remembering the people who were killed and injured in New Zealand; to stand with my community, the Muslim community, at a time when we feel very vulnerable - when we feel like we've been punched in the gut in a big way.
"For these people who are victims in these mosques - they are refugees, they are people who are from our migrant communities who've chosen to live in New Zealand, thinking they would find a safe place where they could be free in their religion and their culture".
The response from some Muslim countries went further, blaming politicians and the media for stoking that hatred.