The inquiry would look at what could have or should have been done to prevent the attacks.
It would look at the alleged gunman and his activities before the attacks.
It would look at the SIS, GCSB, Police, Customs, Immigration and any other relevant agencies.
It would look at events leading up to the attack but not the immediate response. The work of first responders would be looked at separately.
Ardern said the precise terms of reference, including who would lead the inquiry and its duration, would be finalised within a fortnight.
"What I can say today is that there will be a focus on whether our intelligence community was concentrating its resources appropriately and whether there were any reports that could or should have alerted them to this attack."
No stone would be unturned to get to the bottom of what happened and whether there were any opportunities to prevent it.
"I want recommendations on how any such attack in the future can be stopped," she said.
Ardern said Royal Commissions were reserved for matters of the gravest public importance and the mosque shootings was clearly one of those.
New Zealand was not a surveillance state but Ardern said questions needed to be answered around whether the activities of an individual should have been known about.
"The agencies themselves are welcoming independent oversight and investigation into that very question."
Ardern wouldn't give a timeframe yet on the duration of the inquiry. People wanted answers and shouldn't wait a long time for those, but the inquiry needed time to do its job properly, she said.
"We're weighing all of that up."
What is a Royal Commission of Inquiry?
A Royal Commission is the most serious response available to the Government and investigates matters of "great importance and difficulty".
It engages in fact-finding and preventing future recurrences. It investigates why the situation came about and then recommends policy or legislative changes to prevent it happening again.
The Government establishes terms of reference, which set out what subjects are to be investigated. However a Royal Commission is independent from the Government and reports to the Governor-General.
The way the inquiry is conducted and the commission is run is decided by the chairperson and commissioners. The Government cannot interfere in the direction taken by an inquiry, or influence the findings.
A Royal Commission can inquire into any matters it sees fit in order to determine the cause of the issues.
Chaired by a High Court judge who is appointed by the Governor-General, it has the powers of compulsion in regard to witnesses, documentation and awarding costs. This enables the Royal Commission to uncover information which might otherwise be difficult to obtain.
Evidence is gathered from a range of different places and sources, including from participants and through the commission's own investigations. Public hearings are one important part of the inquiry process. They provide an opportunity to clarify matters, test disputed material and ensure that key evidence is discussed in public.
A Royal Commission is not able to determine legal rights and liabilities. Findings and recommendations are not binding upon any party, including the Government.
Credit DIA
Current and Previous Royal Commissions include:
• Royal Commission of Inquiry into Historical Abuse in State Care and in the Care of Faith-based Institutions (Ongoing)
• The Canterbury Earthquakes Royal Commission (2012)
• Royal Commission on the Pike River Coal Mine Tragedy (2012)
• Royal Commission on Auckland Governance (2009)
• Royal Commission on Genetic Modification (2001)
