New Zealand on Thursday (March 21) announced an immediate ban on military-style semi-automatic and assault rifles.

Prime Minister Jacinda Ardern announced the ban as part of sweeping changes to the nation's gun laws in the wake of last Friday's attacks on two mosques in Christchurch that left 50 people dead and dozens wounded.

At a press conference, she said the terror attack highlighted the weakness of the nation's gun laws.

“I am announcing that New Zealand will ban all military-style semi-automatic weapons. We will also ban all assault rifles,” Ms Ardern said, while announcing interim measures that will stop a rush of purchases before legislation on the measures takes effect.

She added that high-capacity magazines and devices similar to bump stocks – which make rifles fire faster – will also be banned.

“In short, every semi-automatic weapon used in the terrorist attack on Friday will be banned in this country,” she said.

For the guns that are still out there, Ms Ardern announced a buy-back scheme that will cost between NZ$100 million and NZ$200 million (S$92 million and S$185 million), depending on the number of weapons received.

"(To) the current owners of the weapons we have moved to ban, I acknowledge that many of you will have acted within the law,” Ms Ardern said.

“In recognition of that and to incentivise their return, we will be establishing a buy-back scheme.”

Anyone who keeps the guns after an amnesty period will face fines of up to NZ$4,000 and three years in jail.

Ms Ardern brushed aside suggestions of opposition to the ban.

“The vast majority of New Zealanders will support this change. I feel incredibly confident of that,” she said.

The lone gunman who carried out last Friday's attacks was armed with semi-automatic rifles including an AR-15.

Australia banned semi-automatic weapons and launched a gun buy-back after the Port Arthur massacre in 1996 in which 35 people were gunned down. The AR-15 was used at Port Arthur. It has also been used in a number of high-profile United States mass shootings.

Ms Ardern said that similar to Australia, the new gun laws will allow for strictly enforced exemptions for farmers to conduct pest control and animal welfare.

Federated Farmers, which represents thousands of farmers, said it supported the change.

"This will not be popular among some of our members, but after a week of intense debate and careful consideration by our elected representatives and staff, we believe this is the only practicable solution,” Federated Farmers Rural Security spokesman Miles Anderson said in a statement.

New Zealand, a country of less than five million people, has an estimated 1.2 million to 1.5 million firearms, around 13,500 of them military-style semi-automatics. The minimum legal age to own a gun in New Zealand is currently 16, or 18 for military-style semi-automatic weapons.