When asked why they weren't on any lists, Ardern said: "It's an indication they had not acted in a way that warranted it."
She said she was not in a place to provide additional information beyond that.
But she would "absolutely" be raising questions as to why these individuals weren't on any watch lists with her officials.
Ardern said it was her understanding that those in custody were not on a watch list in Australia either.
But she stressed it was early days – "that's the assurance I can give people at this time. The people are in custody, obviously being questioned".
Asked if there had been too much attention given in the past to Muslim terrorist risks rather than that of the risk of white supremacy, Ardern said: "I think we should be vigilant of extreme ideology and extreme violent acts".
She said the focus of New Zealand's security agencies have been monitoring extremism "regardless of where it comes from".
"We need to be blind in that regard – we need to absolutely focus on threat and ideology and extremism because obviously, that is what we have experienced today."
Ardern said the attacks appeared to be well planned.
Four suspects have been apprehended – three were connected to the attack.
Ardern said they were people she would describe as having "extremist views that have absolutely no place in New Zealand, or in fact, have no place in the world".
New Zealand's national security threat level has been raised from low to high, Ardern said.
This is the first time in New Zealand's history that the country's threat level has been raised to this level.
This was to ensure all New Zealand's intelligence agencies were responding in the most "appropriate way" – including at the border.
Ardern said New Zealand was not a target because it is a safe harbour of those who hate, nor because New Zealand condones racism or is an enclave for racism.
"We were chosen for the very fact that we are none of these things."
She said New Zealand represents kindness, compassion and a home for those who share New Zealand's values.
"Those values, I can assure you will not and cannot be shaken by this attack."
Speaking directly to the killers, Ardern said: "You may have chosen us, but we completely reject and condemn you."
