A Christchurch gun shop on Monday acknowledged selling guns online to the 28-year-old white supremacist accused of killing 50 people in mosque shootings that have upturned New Zealand's reputation as among the world's most tolerant and safe nations.

At a news conference, Gun City owner David Tipple said the store sold four guns and ammunition to Brenton Harrison Tarrant through a 'police-verified online mail order process.' The store 'detected nothing extraordinary,' about the buyer, he said.

Separately, New Zealand's Prime Minister Jacinda Ardern said gun law reforms would be announced within 10 days and an inquiry conducted into intelligence and security services who failed to detect the risk from the attacker or his plans. There have been concerns that intelligence agencies have been overly focused on the Muslim community in detecting and preventing security risks.

Gun City owner David Tipple at a press conference in Christchurch on Monday. AP

Police commissioner Mike Bush said the police are certain Tarrant was the only gunman but aren't ruling out that he had support.

'I would like to state that we believe absolutely there was only one attacker responsible for this,' he told a news conference. 'That doesn't mean there weren't possibly other people in support and that continues to form a very, very important part of our investigation.'

None of the guns sold to Tarrant were military-style, semi-automatic weapons, according to Tipple. It was not clear if any of the firearms Tarrant purchased from Gun City were used in Friday's shootings.

In vowing to tighten gun laws, Ardern has said the attacker used five guns, two of them semi-automatic, which were purchased with an ordinary gun license and modified.

Tipple said he felt no responsibility for the tragedy and refused to say whether he believed gun ownership laws should change in New Zealand, insisting a debate over guns should be held at another time.

'This man wrote in his manifesto that the purpose of using a firearm was to divide us,' Tipple said. 'If we allow him to make changes in our ideology, in our behaviour, he's won.'

His store has been criticised for leaving out a roadside advertising billboard that shows a parent helping children with rifle target practice in the wake of the shootings.