Speaking at a press conference on Sunday following the attacks that killed 50 people, Ms Ardern said if her office could have done anything with the information sent by accused shooter Brenton Tarrant, "absolutely we would have".
Ardern said by the time a police plan was in place, the 911 number was already buzzing, shots had been fired.
Greg Robertson, head of surgery at Christchurch Hospital said staff were used to seeing gunshot wounds and severe injuries, but the scale and nature of the attacks was different.
Brenton Tarrant, a 28-year-old white supremacist obsessed with the Crusades, toured the Continent and came to Britain to stay for up to a fortnight in 2017.
"At just that moment, there was one young guy who usually takes care of the mosque and helps with parking and other stuff, so (the man) saw an opportunity and he pounced over to him and grabbed his gun".
One additional victim was found at the Dean Ave Mosque.
Police officers search the area near the Masjid Al Noor mosque, site of one of the mass shootings at two mosques in Christchurch, New Zealand, Saturday, March 16, 2019. "Just helping people is his main thing".
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". "Unfortunately, we can't claim that New Zealand is a safe place anymore", spokesman for Syrian Solidarity New Zealand Ali Akil told reporters.
A fourth person was arrested yesterday in the wake of the shootings, but has been released without charge.
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When asked whether this was a normal response time, Commissioner Bush said it met expectations.
Hungry for any news, families and friends of the victims gathered at the city's Hagley College, near the hospital.
Most of the victims of the attacks were immigrants from Pakistan, India, Malaysia, Indonesia, Turkey, Somalia and Afghanistan. "He liked to do marriages - he married me and my wife", said Sulaman Abdul, who fled Somalia as a refugee in 1993 for New Zealand.
His relatives in the Australian town of Grafton, in New South Wales, contacted police after learning of the shooting and were helping with the investigation, local authorities said.
There are an estimated 1.5 million firearms in New Zealand, which has a population of only 5 million, but it has had low levels of gun violence. Ardern suggested "now is the time for change" and hinted she was closely looking at rules regulating ownership of semi-automatic weapons.
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.
He also reached out to the Muslim community in Christchurch and in New Zealand.
Arden used some of her strongest language yet about gun control, saying that laws need to change and "they will change".