Prime Minister Jacinda Ardern has expressed sorrow about the attack at a synagogue in San Diego where the gunman reportedly said he had been inspired by the Christchurch massacre.

"Our hearts go out to the Jewish community," she said through a spokeswoman.

"To see an attack while people are at worship is devastating.

"New Zealand condemns all acts of terrorism and violence and stands for freedom of religion and the right to worship safely.

Advertisement

"New Zealand's resolve to drive global action to eliminate violent extremism has been strengthened by the terrorist attack on our soil on March 15."

The synagogue shooting left one woman dead and three people, including the rabbi, injured.

NBC has reported the gunman posted a letter on a far-right website claiming to be inspired by the Christchurch gunman who murdered 50 people in two mosques on March 15.

Asked if it the latest attack on worshippers lent greater urgency to her mission to Paris in May where she and President Emmanuel Macron will co-chair a meeting to eliminate violent extremism and the incitement of terrorism from social media, she gave a general answer.

"The Paris Summit is a call for collective, urgent action by Governments and tech companies to eliminate violent extremist content online.

"This is a different approach and New Zealand is taking a leadership role because our resolve is strong to ensure social media cannot be used again the way it was in the March 15 terrorist attack."

The gunman is reportedly a 19 year old student from San Diego and is being held in custody.

Reports say the gunman said in a letter that he was 'inspired' by the Christchurch mosque shootings on March 15.

NBC reports that an anti-Semitic "open letter" posted in the name of the San Diego suspect was posted on far-right message board 8chan before the attack.

NBC reports: "The 8chan post appears near identical to the one left hours before opening fire at a mosque in Christchurch. It even links to the same document dumping sites to host the open letter".

The shooting came six months after a gunman killed 11 at a synagogue in Pittsburgh, the deadliest attack on the Jewish community in US history.

The Christchurch and Pittsburgh suspects are listed as a direct inspiration in the letter, NBC reports.

Speaking about the San Diego shooting, Opposition leader Simon Bridges said: "I condemn this cowardly attack on people of faith worshipping their God.

"It is the same thing we have seen in Christchurch and Sri Lanka and now in San Diego.

"I'm sure all New Zealanders will stand against what has happened in the US."

About 250 people were killed in Sri Lanka in eight bombings on Easter Sunday in churches and hotels.

Mosques in New Zealand are still being guarded by armed police for fear of copy-cat attacks, although the terrorism threat level has been reduced from high to medium.

Ardern said the national terrorism threat level was under continual review by CTAG - an independent multi-agency group.

"Keeping New Zealanders safe in their communities remains our top priority."