Bush said they were "dealing with a very serious and tragic series of events in the Christchurch Canterbury area" and warned people to stay off the streets.
According to a report from Radio NZ, Adam Ali was praying at the Linwood Masjid when a gunman entered the mosque and opened fire.
The alleged killer is believed to have live-streamed the shooting on Facebook.
Police in the city of Christchurch on New Zealand's South Island have responded to shootings at two local mosques.
Police have not confirmed deaths or injuries yet. "The person who has perpetuated this violence against us is not. Three are men and one is a woman, we are working through that as well", NZ police commissioner Mike Bush said.
"Police are responding with its full capability to manage the situation, but the risk environment remains extremely high", Bush said in the statement.
"The gun was (at the) front door, I run behind the mosque and I was sitting behind... and I called the police and the police take a long time".
New Zealand police deployed armed officers in central Christchurch on Friday after reports that several shots had been fired, police said. More information will be added as soon as it becomes available.
Koroi who lives in Hornby with his family says they are just 15 minutes away from where the shooting took place.
"I can see a girl dead in the middle of the road", Ibrahim said in tears.
"I would describe it as an unprecedented act of violence, an act that [has] absolutely no place in New Zealand", Ms Ardern said.
Three local schools are now on lockdown.
A witness told the Associated Press many people have been killed in a mass shooting at a mosque.
The publication also reported that the Bangladeshi Cricket Team had just left the mosque in Christchurch before the attack.
Ms Jamieson told SBS News bloodied victims lay in ambulances and shaken victims stood comforting each other following the gunman's shooting.
One person is in custody and there have been people killed in two mosques, one on Deans Ave and one in Linwood.
In a 74-page manifesto published online he outlined who he was and why he carried out the massacre at the Christchurch mosque. "Frightening experience and please keep us in your prayers", Bangladesh cricketer Tamim Iqbal said.
This is a developing story.