Police have said the man - who can not yet be named - was not involved in the attack in Christchurch allegedly carried out by Australian white supremacist Brenton Tarrant, 28, on Friday. The main shooting happened at Al Noor Mosque in a suburb west of the city center, where 42 people were killed.
"We believe absolutely there was only one attacker responsible for this", Bush said at a news conference.
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".
Following the attack Tarrant was charged with murder and a New Zealand court ordered that he was to remain in custody until 5 April. One other person detained in the investigation faces charges of inciting racial hatred, and another faces a firearm charge, but police say neither was involved in the attacks.
Australian police searched two homes in New South Wales thought to be linked to Friday's shootings at Christchurch mosques as New Zealand's Prime Minister Jacinda Ardern said her government had agreed in principle to introducing tougher gun laws.
Dutton said Australia's security agencies are very concerned about retaliatory or copycat attacks in the wake of the Christchurch terror attack.
"I will announce a more fulsome set of details once we have worked through the decisions the Cabinet made today", Ardern said, adding that concrete proposals will be announced within 10 days. However, none of the items bought were semi-automatic weapons and it remains unclear if the weapons bought were used in last Friday's attack.
Australia introduced some of the world's toughest gun laws after its worst mass killing, the 1996 Port Arthur massacre in which a lone gunman killed 35 people using a USA staple of mass shootings in that country - the semi-automatic AR-15, the same weapon used in the Christchurch terror attacks.
"It is a stringent process at some level, but in saying that 99.3 percent of all applicants get their gun license approved", New Zealand Police Association President Chris Cahill explained to Xinhua.
Facebook says it has deleted more than 1.5 million copies of the video of the mosque attacks in New Zealand in the first day after the incident.
Police have said the teenager - who can not yet be named - was not involved in the attack on two mosques. Her declarations have been celebrated by many in New Zealand, some of whom had no idea that military-style semiautomatic weapons were so prolific in a country famously known for its extremely low murder rate.
Facebook in response said that it removed a total of 1.5 million videos from the platform within 24 hours. The weapons seen in a video of the massacre that Tarrant live-streamed had much larger magazines.
Now even the advertising of weapons is in question, with anger focussed on Gun City's billboard on a busy highway showing a man teaching two children how to shoot. "But it is at the discretion of the family", said Mo, who asked to be identified by just one name. He said there would be no burials on Monday.