Prime Minister Jacinda Ardern has a strong message from New Zealand to the gunmen who carried out the country's worst attack – "You may have chosen us, we utterly reject and condemn you".

Ardern addressed the nation this evening following a briefing with top officials on what she called a terrorist attack on two mosques in Christchurch.

Security has been beefed up at the borders and across the country following the shootings.

Four people have been arrested, three of them suspected killers and another unconnected to the trio.

"These are people who I would describe as having extremist views that have absolutely no place in New Zealand and in fact have no place in the world," Ardern said.

Ardern said the country's thoughts and prayers were with the people targeted in the attacks.

"Christchurch was their home. For many, this may not have been the place they were born, in fact for many, New Zealand was their choice - the place they actively came to, and committed to; the place they were raising their families; where they were parts of communities that they loved and who loved them in return.

"It was a place that many came to for its safety. A place where they were free to practice their culture and their religion," Ardern said.

She said New Zealand was not targeted because it was a safe harbour for people who hated.

"We were not chosen for this act of violence because we condone racism, because we are an enclave for extremism. We were chosen for the very fact that we are none of those things.

"Because we represent diversity, kindness, compassion. A home for those who share our values. Refuge for those who needs it. And those values will not and cannot be shaken by this attack."

Ardern also outlined increased security measures in Christchurch and around the country.

"While we do not have any reason to believe at this stage that there are other suspects, we are not assuming that at this stage."

The Defence Force was transporting more police to Christchurch and the 
national security threat level had been lifted from low to high.

"This, I want to assure people, is to ensure that all our agencies are responding in the most appropriate way. That includes at our borders.

"There is heightened security, so we can assure people of their safety and the police are working hard to ensure people are able to move around their city safely," Ardern said.