Dark days for NZ with mosque terror: PM

New Zealand Prime Minister Jacinda Ardern says 40 people have been killed and another 20 seriously injured in terror attacks on two mosques in Christchurch.

"We believe that 40 people have lost their lives in this act of extreme violence," she said on Friday.

"Ten have died at Linwood Avenue mosque. Three of which were outside the mosque itself."

"A further 30 have been killed at Deans Avenue mosque."

She called it one of New Zealand's darkest days and stressed there was no place for extremist violence in her country, or perpetrators of it.

Three men and a woman have been apprehended by police.

"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," the prime minister said.

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

She told New Zealanders they were not chosen for this act of violence because we condone racism but because they represent diversity, kindness, compassion and a refuge for those who need it.

"And those values I can assure you will not and cannot be shaken by this attack," she said.

"You may have chosen us, but we are utterly reject and condemn you," she said of the extremist attackers, none of whom were on a security watchlist.

She said there police have got no reason to suspect there might be others out there, but they have not ruled it out.

She said New Zealanders would be shocked by the violence and urged people not to give any oxygen to the message of hate behind it

The mosques and a number of schools are in lockdown and police were saturating the areas with staff.

Ms Ardern said those attacked should have been safe.

Many of those who were killed or injured may be migrants to New Zealand, even refugees, she said.

"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."

Australian-born Brenton Tarrant had issued a 74-page "manifesto" saying he carried out the fatal shootings at Christchurch mosques to avenge the deaths of thousands in Europe.

He described himself as "Just an ordinary White man, 28 years old. Born in Australia to a working class, low income family."

Under a heading "Why did you carry out the attack?", he said it was to avenge "thousands of deaths caused by foreign invaders".

New Zealand Police Commissioner Mike Bush warned people not to go to any mosque, anywhere in New Zealand, on Friday.

"It is a very serious and grave situation," he said.

It began when a gunman opened fire at a mosque at 1.40pm local time, with witness saying there may have been up to 500 people.

© AAP 2019