"There is no place is New Zealand for such acts of extreme and unprecedented violence which, it is clear, this act was.
"For now my thoughts and, I'm sure, the thoughts of all New Zealanders, are with those who have been affected and also with their families.
"My thoughts also to those in Christchurch who are still dealing with an unfolding situation."
Ardern said it had occurred in a place were expressing their religious freedom "where they should have been in a safe environment, and they have not been today.
"The people who were the subject of this attack today, New Zealand is their home, they should be safe here. The person who has perpetuated this violent act against them, they have no place in New Zealand society."
Ardern urged people in Christchurch to follow police advice and stay indoors.
"I acknowledge that may mean some families are separated," she said.
She said people should be reassured that police were managing the situation and Christchurch Hospital was dedicated to treating the injured.
Ardern was in New Plymouth for a number of events today but cancelled them as the news broke.
She is flying back to Wellington, where she said agencies were already gathering.
The head of the Department of Prime Minister and Cabinet, and Ardern's acting Chief of Staff are meeting at Police Headquarters in Wellington.
Ardern is due to arrive in Wellington before 6pm and will then attend a briefing from wider agencies and the Officials' Committee for Domestic and External Security Co-ordination (ODESC).
After that, she is expected to hold a further media briefing at Parliament.
Speaking to Newstalk ZB, Finance Minister Grant Robertson said it was too early to say if there had been any intelligence prior to the attack but said he would be briefed by NZSIS and security personnel soon.
"When the Prime Minister gets back to the Beehive, we will be forming together all of the key officials and ministers to go through the situation as we know it so we get a full briefing both on what has happened, and what is being done."
He would not comment on any specifics regarding the shooter while the situation was "still evolving" but said there would be a time in the very near future where those details will be explored.
Officials Committee for Domestic and External Security Coordination
The Officials Committee for Domestic and External Security Coordination – commonly referred to as ODESC - is a committee of chief executives which manages national security in New Zealand in both its governance and its response mode.
During an emerging or actual security event ODESC:
• Provides all-of-government co-ordination at the chief executive level of the issues being dealt with through the response
• Provides strategic advice on priorities and mitigation of risks beyond the lead agency's control
• Ensures that the lead agency and those in support have the resources and capabilities required to bring the response to an effective resolution
• Provides the linkages to the political level, including supporting ministers to make decisions about strategic policy, authorisation of resources or any other decisions which sit within ministers' area of control
• Exercises policy oversight and advises the Prime Minister, Cabinet and, when activated, the Cabinet National Security Committee, accordingly.
