"I want to assure the public we are doing everything we can to make sure there is no more harm to their communities", Bush said.
Four people have been detained, and one is Australian. "The persons who has perpetuated this violence against us. have no place in New Zealand". He described the gunman as "an extremist, right-wing, violent terrorist".
Three others remain in custody.
Reports attacks occurred at local mosques.
Mr Sayed, who has been living in New Zealand for over 10 years, told Indian broadcaster NDTV that he had witnessed the man "creep up behind the shooter and hold him until his gun dropped".
The foreign ministry says there are 330 Indonesian citizens in Christchurch, 130 of them students.
Police Commissioner Mike Bush did not elaborate on the suspects who are in custody.
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A second witness, Faisal Sayed, also described seeing the mosque worker's heroic actions. "Three of which were outside the mosque itself".
"This is one of New Zealand's darkest days", she said.
In the video that was apparently livestreamed, the gunman spends more than two minutes inside the mosque spraying terrified worshippers with bullets again and again, sometimes firing at people he has already cut down. Police also said a number of improvised explosive devices were discovered on vehicles in the immediate area.
A gunman entered a packed mosque on Deans Ave, in Christchurch, about 1.40pm on Friday and opened fire.
"This goes to the seriousness of the situation", Bush said.
The person further claimed that the injured Malaysian was still at one of the mosques in Christchurch that gunmen attacked today.
No details were immediately available.
"New Zealand's terrorist factor right now is low, which means compared to other countries we are one of the safest countries in the world, so it is unlikely the authorities saw this coming". People are urged to stay indoors and report any suspicious behaviour.
Sabir Hussain told Stuff he was praying at the same mosque when he heard the imam (leader of the mosque) say "our brothers have been shot". "It's a small group of people that have very, very serious problems".