Australian-born Brenton Tarrant, two other men and a woman were arrested after at least one gunman opened fire on worshippers as they gathered for Friday prayers, with unconfirmed reports of dozens of deaths.

A number of car bombs were also disengaged by military personnel after the shooting.

Asked if one of the attackers had been wearing explosives, police commissioner Mike Bush said they had "been made safe."

Police said they were still treating the event as ongoing and urged residents to stay indoors.

They have asked mosques around the country to close their doors.

Armed police descended on the Masjid Al Noor mosque, near Hagley Park, after shooting broke out at 1.40pm (NZT) on Friday, with public facilities locked down and the area cleared out.

Reports later emerged of shots at another mosque in Linwood Avenue.

"I can confirm that the individual who was taken into custody I have been advised is an Australian-born citizen," Australian prime minister Scott Morrison told reporters in Sydney on Friday.

A witness, with blood splatters across his shirt, told AAP the shooter changed magazines seven times after opening fire as the crowd gathered to pray.

"When the shooting started people started rushing out, and the door was closed, and the guys came on them and started shooting them," he said, describing how he hid under a bench and pretended to stop breathing.

"He went to all the different [rooms] and he shot everyone."

Several hundred people were inside and witnesses reported seeing "blood everywhere".

Others described seeing as many as 40 people injured.

"I heard a big sound of the gun. And a second one, I ran. Lots of people were sitting on the floor," one told TVNZ.

"The floor. There's lots of blood on the floor, you can see when you go in."

A 17-minute video that appeared to show a shooting, taken from a helmet camera, had been posted online with police urging the public not to share it.

Telecommunications companies were also working to have the video taken down or blocked.

A man named as Tarrant and claiming to be a 28-year-old Australian has issued a 74-page "manifesto" saying he carried out the fatal shootings at Christchurch mosques to avenge the deaths of thousands in Europe in terror attacks.

His manifesto, titled "The Great Replacement", was posted on a message board website.

Police have declined to say if the incident is being treated as terrorism, whether the arrested group were known to police or to comment on the possible motivation.

Video posted on social media showed police ramming a vehicle and removing a person inside.

Prime Minister Jacinda Ardern said called it one of her nation's darkest days.

"I would describe it as an unprecedented act of violence, an act that has absolutely no place in New Zealand," she said.

Christchurch Mayor Lianne Dalziel urged residents to pull together.

"I never could believe that something like this would ever happen in the city of Christchurch, but actually I would never believe that this would ever happen in New Zealand," she said.

New Zealand's top national security agencies held an emergency meeting in the afternoon.

Players from the Bangladesh cricket team, in Christchurch to play New Zealand in a Test match starting on Saturday, were in the mosque narrowly escaped the attack. They had been left shaken but uninjured, their coach told media.

The match since been cancelled.

Australian Associated Press