Forty people were killed and more than 20 seriously wounded in mass shootings at two mosques in New Zealand's Christchurch on Friday, Prime Minister Jacinda Ardern said.

Ardern said New Zealand had been placed on its highest security threat level. She said four people in police custody held extremist views, but had not been on any police watchlists.

"This is one of New Zealand's darkest days," said Prime Minister Jacinda Ardern. "Clearly what has happened here is an extraordinary and unprecedented act of violence."

Witnesses told media that a man dressed in a military-style, camouflage outfit, and carrying an automatic rifle had started randomly shooting people in the Al Noor mosque.

The Bangladesh cricket team was arriving for prayers when the shooting occurred but all members were safe, a team coach told Reuters.

New Zealand's Police Commissioner Mike Bush said the police had one person in custody but they were not sure if others were involved, and people should stay away from mosques.

Police said earlier they were hunting "an active shooter" in the centre of Christchurch city.

"A serious and evolving situation is occurring in Christchurch with an active shooter," New Zealand Police Commissioner Mike Bush said.

"Police are responding with its full capability to manage the situation, but the risk environment remains extremely high."

Media reported that a gunman opened fire inside a mosque in Christchurch's Hagley Park district. There were reports of armed police at a second mosque in the suburb of Linwood.

People in centre of the city should stay indoors, police said. All Christchurch schools and council buildings have been placed into lockdown.

Radio New Zealand quoted a witness inside the Al Noor mosque saying he heard shots fired and at least four people were lying on the ground and "there was blood everywhere".

"Horrified to hear of Christchurch mosque shootings. There is never a justification for that sort of hatred," said Amy Adams, a member of parliament from Christchurch.

The Bangladesh cricket team is in Christchurch to play New Zealand in a third cricket test starting on Saturday.

"They were on the bus, which was just pulling up to the mosque when the shooting begun,” Mario Villavarayen, strength and conditioning coach of the Bangladesh cricket team, told Reuters in a message.

"They are shaken but good.”

Muslims account for just over 1 percent of New Zealand's population, a 2013 census showed.

"Many of those who would have been affected by this shooting may be migrants to New Zealand," Ardern said.

"They may even be refugees here. They have chosen to make New Zealand their home and it is their home ... they are us. The persons who has perpetuated this violence against us ... have no place in New Zealand."