The main suspect in mass shootings at two New Zealand mosques was charged with one count of murder a day after an attack that killed 50 people and wounded dozens, prompting the prime minister to vow reform of the country's gun laws.

Brenton Harrison Tarrant, a 28-year-old Australian citizen, appeared in a Christchurch District Court on Saturday and was remanded without a plea until his next scheduled appearance in the South Island city's High Court on April 5.

Handcuffed and wearing a white prison suit, Tarrant did not speak. His court-appointed lawyer made no application for bail or name suppression.

He was likely to face further charges, police said.

The attack, which Prime Minister Jacinda Ardern labelled as terrorism, was the worst peacetime mass killing in New Zealand and the country raised its security threat level to the highest.

Ardern said on Saturday that Tarrant intended to continue the rampage before he was caught by police.

“The offender was mobile, there were two other firearms in the vehicle that the offender was in, and it absolutely was his intention to continue with his attack,” Ardern told reporters in Christchurch.

Tarrant has been identified as a suspected white supremacist, based on his social media activity.

Footage of the attack on one of the mosques was broadcast live on Facebook, and a “manifesto” denouncing immigrants as “invaders” was also posted online via links to related social media accounts.

The video footage showed a man driving to the mosque, entering it and shooting randomly at people inside. Worshippers, possibly dead or wounded, lay on the floor, the video showed. Reuters was unable to confirm the footage's authenticity and police urged people not to view or share it.

Police said the alleged shooter was arrested in a car, which was carrying improvised explosive devices, 36 minutes after they were first called. It was still unclear whether any other shooters were involved in the attacks.

Two other people were in custody and police said they were working to understand their involvement.

Armed police were deployed at several locations in all cities, unusual in a country that has had low levels of gun violence.

Ardern said the main suspect was a licensed gun owner who used five weapons during his rampage, including two semi-automatic weapons and two shotguns.

Authorities were working to find out how he had obtained the weapons and a license, and how he was able to enter the country to carry out the attack, she said.

“I can tell you one thing right now, our gun laws will change,” Ardern told reporters, saying a ban on semi-automatic weapons would be considered.

None of those arrested had a criminal history or was on any watch list in New Zealand or Australia.