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.
After Tarrant left, the judge said that while "there is one charge of murder brought at the moment, it is reasonable to assume that there will be others".
Australian police say the family of Tarrant is helping their investigation.
"The United States strongly condemns the attack in Christchurch".
- AP Police stand by a collection of flowers near the Linwood Mosque in Christchurch, New Zealand, Saturday, March 16, 2019.
Cook Islands prime minister Herny Puna said his country's prayers first and foremost were with the victims of the shooting, "their families, friends and loved ones whe are now faced with the irreconcilable loss of their loved ones".
The city is on edge as helicopters are heard overhead and people are advised to stay indoors. Two Indians have died in the attacks, a third person is battling for his life, Asaduddin Owaisi, the leader of AIMIM (All India Majlis-e-Ittehadul Muslimeen Party) said.
A noisy generator raring from a construction site next to the Christchurch Justice Precinct was a stark and irritating reminder of those tragic events, as media and members of the public gathered awaiting Tarrant's court appearance.
"I can tell you one thing right now, our gun laws will change", Ms Ardern said in Wellington.
At least 49 people were killed in the attacks on two mosques.
New Zealand Prime Minister Jacinda Ardern on Saturday promised to reform the country's gun laws.
A second survivor, Faisal Sayed, said: "If that hadn't happened, many more would have died and I wouldn't be here now".
Ardern noted that this individual traveled around the world and spent "sporadic periods of time" in New Zealand.
We know that from this devastation and deep mourning, the people of New Zealand will unite to show that such evil can never defeat compassion and tolerance.
"It is clear that this can now only be described as a terrorist attack", said Ardern.
"The terrorist attack appeared to have been carried out by a white nationalist who posted a racist manifesto online and streamed live video of the killings on Facebook", reports The New York Times. Armed police were deployed at several locations in all cities, unusual in a country where levels of gun violence are low.
Police recovered two semi-automatic weapons, two shotguns and a lever-action firearm after the attacks. Tarrant had a category-A gun license which meant he could legally buy the weapons he used, although there are suggestions the guns were altered to make them more lethal, Ardern told reporters.
Officers arrested two others following the attacks and are now working to establish whether they had any involvement in the terror attacks against the Muslim community in New Zealand.