The massacre of dozens in Christchurch mosques has been described as an "act of terror .
Brenton Tarrant, 28, on Saturday appeared in court on a charge of murder after an unprecedented killing spree across two mosques left 49 worshippers dead and dozens of others, including children, badly injured.
The former NSW personal trainer, wearing a tunic and handcuffed, did not apply for bail and has been remanded in custody without plea until April 5.
Two armed guards escorted Tarrant into the Christchurch court and he showed no expression as District Court Judge Paul Kellar read one charge of murder to him.
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".
The attack on Friday, confirmed as an act of terror by officials, is the worst in the country's history and has seen its threat level raised to high for the first time.
Police on Saturday said they would keep guarding mosques around the country until further notice. Helicopters remained hovering overhead in Christchurch, while heavily-armed officers patrol streets.
Officials say 41 people were killed at the Masjid Al Noor mosque on Deans Avenue, before shooting broke out at Linwood Masjid six kilometres away, leaving seven more dead. One later died in hospital.
Dozens remain in hospital, with at least 12 - including a four-year-old girl - in a critical condition.
Police are still piecing together the exact series of events, and expected to clarify the charges and involvement of two others arrested.
"There is one prime suspected perpetrator of this attack, but police are insuring they include an investigation to give assurance there are no ongoing threats," Ardern told reporters.
She also praised the efforts of police who stopped the gunman in his car, after criticism about the 36 minutes it took to arrest him.
"The offender was mobile ... and it absolutely was his intention to continue with his attack."
Ardern flew to Christchurch on Saturday, holding a series of meetings to assure the Muslim community their safety was top priority.
"This is not New Zealand," she told a group at Christchurch's refugee centre.
"This act of terror was brought to our shores and rained down upon us."
Ardern vowed New Zealand would be changing its gun laws, after it was discovered Tarrant was licensed and had five guns, some modified.
The Australian gunman, who had based himself in Dunedin, used two semi-automatic weapons, two shotguns and a lever-action firearm, Ardern said.
"I think people will be seeking change, and I'm committing to that."
Tarrant grew up in Grafton in NSW and stated in a rambling 74-page "manifesto" posted online before the attack that he had spent years in planning vengeance for deaths in Europe, before deciding three months ago on Christchurch.
Questions have been raised about why none of those arrested had appeared on a watchlist of New Zealand or Australian security agencies.
Thousands are expected to attend vigils in Auckland on Saturday night, while others are planned throughout the country over the next week.
Flowers are being left at cordons near the attack sites in Christchurch.
"If this evil thinks we will stop going to our mosque here or stop doing our worship to our god, Allah, we cannot ever stop," Linwood mosque Imam Ibrahim Abdelhalim told reporters.
"We are part of the New Zealand community."
Leaders across the world, including Queen Elizabeth, British Prime Minister Theresa May, Pope Francis and United States president Donald Trump have sent condolences and condemned the attack.
Prime Minister Scott Morrison - who has been in contact with Ardern - said the "real enemy was hatred and intolerance - this is the root of all extremism and terrorism".
"As Prime Minister Jacinda Ardern said yesterday, an attack against one is an attack against all of us, from all communities who love peace, who love the freedoms that we have both here in Australia and New Zealand and the peace-loving peoples of the world," he said in Sydney on Saturday.
Opposition leader Bill Shorten made an impassioned plea to stop giving "oxygen" to the "evil jokers at the margins" who perpetrate hate speech.
© AAP 2019