He said the arrest of Australian man Brenton Tarrant - who on Saturday faced court on one count of murder - was risky.
"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.
"Charges have been laid, we can expect additional charges will be appearing in the high court on the fifth of April, so there is obviously a process that needs to be gone through here", she said.
"Even if you ask someone about the natural disaster today they will start crying, so this tragedy will stay in the heart of everyone", says Raj Sandhu, 25, who moved to New Zealand from India three years ago, and came to the Al Noor mosque to pay his respects.
Seven were killed at the suburban Linwood Masjid Mosque, and one person died later at Christchurch Hospital.
The number of people identifying as Muslims rose by 28 percent between 2006 and 2013, according to Stats NZ, and just over a quarter were born in New Zealand.
Ardern alluded at a news conference to anti-immigrant sentiment as the possible motive, saying that while many people affected by the shootings may be migrants or refugees, "they have chosen to make New Zealand their home, and it is their home".
Ardern said she had spoken to Trump, who had asked how he could help.
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Speaking to the media at Lakemba Mosque on Saturday afternoon, Prime Minister Scott Morrison expressed the Australian government's "profound and deepest condolences and sympathies for the horrific and tragic terrorist attacks that took place in New Zealand".
As Ardern revealed deatails about the suspect's weapons and his background, she promised changes to the country's gun laws.
"I am deeply saddened by the terror attack in New Zealand & stand with our Muslim community to condemn this hate-fuelled violence", Mayor John Tory tweeted.
Witness Len Peneha said he saw a man dressed in black enter the mosque and then heard dozens of shots, followed by people running from the mosque in terror.
"There's not many words that I can put to what has happened here because it's so calm and relaxed", he said. Such violence, he says, is "being almost endorsed by political rhetoric around banning immigration of Muslims and criminalizing refugees".
The "primary perpetrator" in the mosque shootings was a licensed gun owner and legally acquired the five guns used in the shootings, Ardern said in an address to the nation broadcast live on Saturday.
Abdulrahman Hashi, 60, a preacher at Dar Al Hijrah Mosque in Minneapolis, told The Washington Post that his 4-year-old nephew was among those killed.
Mr Bush said two of those in custody were arrested at the scene, and that police were working to establish whether they had had any involvement in the incident. I can only imagine now how New Zealanders are feeling. "It's not what we are about", Shamim Kassibawi, a publicist from New Zealand and UAE resident, told Arab News.
Before Friday's shootings in the city of Christchurch, the deadliest attack in the South Pacific country was in 1990, when a person went on a shooting spree in the seaside town of Aramoana that left 13 people dead. The attacks killed 49 people and injured 42 others, of those injured, several remain in critical condition.