She said the royal commission of inquiry would look at issues including semi-automatic weapons, social media and the work of intelligence agencies.
A royal commission of inquiry, New Zealand’s highest form of investigation, is chaired by a high court judge and run independently of the government.
Australian white supremacist Brenton Tarrant, 28, has been charged with murder over the March 15 attack. Ms Ardern said: “In short, the inquiry will look at what could have or should have been done to prevent the attack.
"It will inquire into the individual and his activities before the terrorist attack, including, of course, a look at agencies.”
Some people have criticised New Zealand’s intelligence agencies for focusing too much on perceived threats from Muslim extremists and Left-wing radicals and too little on nationalist groups and white supremacists.
Ms Ardern added: “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.”
A royal commission has the power to compel witnesses to testify and organisations to hand over documents, but it remains up to the courts or government to follow through on any recommendations or findings.