New Zealand Prime Minister Jacinda Ardern at Cashmere High School in Christchurch. (File)

New Zealand Prime Minister Jacinda Ardern announced an immediate ban on the sale of assault rifles and semi-automatics Thursday in a muscular response to the Christchurch terror attack that killed 50 people.

"Today I am announcing that New Zealand will ban all military-style semi-automatic weapons. We will also ban all assault rifles," Ardern said, while announcing interim measures that will stop a rush of purchases before legislation is enacted.

"The effect of this will mean that no one will be able to buy these weapons without a permit to procure from the police. I can assure people that there is no point in applying for such a permit," she said.

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, Ardern announced a buyback scheme that will cost between Nz$100 million and $200 million (between US$69 million and $139 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," Ardern said.

"In recognition of that and to incentivise their return, we will be establishing a buyback scheme."

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

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.