Forty-nine people have died in two mosque shootings in Christchurch, with explosive devices found attached to vehicles of some of those arrested.
Thirty people died at the Deans Avenue mosque, while 10 died at the Linwood Avenue mosque.
Canterbury DHB said 48 patients with gunshot wounds are being treated at Christchurch Hospital.
Ms Ardern said it can only be described as a terrorist act and the national security threat level has been lifted from low to high.
She said this was a well-planned attack, with explosive devices also found attached to vehicles of some of those arrested.
Ms Ardern said it appears there are no other shooters at large, but they cannot assume that.
None of the people arrested were on a terrorist watch list, she said.
"There are currently four individuals who have been apprehended but three are connected to this attack who are currently in custody, one of which has publicly stated that they were Australian born," Ms Ardern said.
Earlier this afternoon Police Commissioner Mike Bush said three men and one woman are in custody. Police cannot confirm whether there are other people involved.
Mr Bush confirmed there were fatalities at two different locations - a mosque at Deans Ave and a mosque at Linwood Ave.
He said he was unsure if other locations were under threat or if other people were involved. Police still do not have the identity of the victims of the shooting. Police are setting up a facility so that people can phone in and check on their loved ones, Mr Bush said.
Police say they are still treating the situation in Christchurch as ongoing, and strongly urge people to stay indoors, and keep safe.
"We have mobilised every police resource in the Canterbury region to respond to this," Mr Bush said.
"There were a number of IEDs attached to vehicles that we also stopped. They've been made safe by the Defence Force but that does go to the seriousness of the situation."
Mr Bush said social media footage of the incidents is very disturbing and police are looking at how to remove it from the public domain.
Police have told mosques nationwide to shut their doors until further notice, as the situation in Christchurch unfolds. The mosques where the shootings occured, in Riccarton and Linwood, remain in lock down.
Police are unsure if other locations are under threat and officers have been mobilised across the country, Mr Bush said.
He warned anyone who was thinking of going to a mosque anywhere in New Zealand today not to go, and for mosques to close their doors until they hear from police.
Christchurch Airport is still open, however some flights have been cancelled. Air New Zealand has cancelled 17 regional turbo prop services this evening as it is not possible to screen customers and their baggage, but jet operations from Christchurch Airport will continue.
Forty-eight patients with gunshot wounds are being treated at Christchurch Hospital after the attacks.
Canterbury District Health Board chief executive David Meates said the patients range from young children to adults with gunshot wounds, with injuries ranging from critical to minor.
He said 12 operating theatres are currently in use and due to the nature of some of the injuries, many people will need multiple surgeries.
There is sufficient capacity and staff at Christchurch Hospital and the police cordon has been lifted from the site, so people who need emergency health care can now attend the Christchurch Hospital Emergency Department, he said.
About 200 family members are on site awaiting news of their family members, Mr Meates said.
A witness said a man wearing a military-style jacket opened fire inside the mosque on Deans Ave.
The head of the Federation of Islamic Federations of New Zealand, Dr Mustafa Farouk, told RNZ that up to 500 people were inside the mosque.
Mr Farouk said he believes members of the Bangladesh cricket team may have been worshipping at the mosque at the time.
He said Friday is a big day for the mosque and there would have been between 300 and 500 worshippers there today.
The Bangladesh Cricket Board confirmed all members of the team are safe in Christchurch.
"All members of the Bangladesh cricket team in Christchurch, New Zealand are safely back in the hotel following the incident of shooting in the city," the BCB's official Twitter account tweeted.
"The Bangladesh Cricket Board is in constant contact with the players and team management."
A witness earlier told RNZ they saw at least five people lying on the ground at the Deans Ave mosque.
"There was blood everywhere."
Christchurch Mayor Lianne Dalziel said police are getting the situation under control but it is critical that people stay away from the central city until further notice.
"This is not something I would ever expect to see in the city and it's not something I would ever expect to see in New Zealand, so it is serious and they [police] are treating it seriously," Ms Dalziel told Checkpoint.
"I am very shocked by what I'm hearing but for me but the priority is getting the message out to people they have to stay safe, stay calm and to provide the reassurance that if they're inside a central city building they must stay there in order to stay safe."
Police are getting the situation under control, but it's critical that people stay away from the central city until further notice, she said.
"All council buildings in the central city are in lockdown and police are requesting central city building owners do the same.
"We're looking after some students who were at the climate change rally. They are safe here and we will keep them here until we are advised by the police they can leave."
Students had come into the council building chanting about climate change which Ms Dalziel had also attended in Cathedral Square to receive a letter being sent to all civic leaders.
"They didn't know [about the shootings]... I was the one that had to tell them there'd been a serious incident and the police were asking them to stay put within the building and that was the advice I gave them.
"It has been nerve-racking for them. But the first piece of advice I gave them was to get on their mobile phones and text their families and tell them they were safe ... so they're busy on their cellphones and talking to people. They're safe here."
Earlier, Ms Ardern said many of those who will have been directly affected by the shooting will be migrants and refugees.
"They have chosen to make New Zealand their home and it is their home. They are us."
"The person who has perpetuated this violence against us is not. They have no place in New Zealand. there is no place in New Zealand for such acts of extreme and unprecedented violence which it is clear this act was," she said.
The Prime Minister has convened a high level committee brought together to respond to serious security events.
The Officials Committee for Domestic and External Security Coordination, or ODECS, is a committee of chief executives which manages national security.
Its role is to co-ordinate the response to serious security events across the whole government and advise on how resources are best deployed.
Police are urging people in central Christchurch to stay indoors and report any suspicious behaviour immediately to 111.
All schools in Christchurch were put into lockdown just after 2pm, but that was lifted at about 6pm and parents were asked to contact their school to find out what arrangements there are for collecting their children.
Deans Ave has been closed between Moorhouse Avenue and Riccarton Road. Motorists are being advised to avoid the area.
The president of the international Muslim Association of New Zealand said his community was in shock.
Wellington-based Tahir Nawaz said the attack on the mosques in Christchurch was not something he ever thought would happen in New Zealand.
"New Zealand is known as a country with loving people and we have a peace and harmony with other religions and people. Having something like this is a big shock.
The Jewish community, and others from the area, have been in touch to offer help, and bringing flowers, Mr Nawaz said.
"They are extending a hand to help," he said.
This article was first published on Radio NZ.