New Zealand will ban military-style semi-automatic and assault rifles under tough new gun laws following the killing of 50 people in the country's worst mass shooting, Prime Minister Jacinda Ardern said on Thursday.

In the immediate aftermath of Friday's shootings at two mosques in Christchurch, Ms Ardern labelled the attack as terrorism and said New Zealand's gun laws would change.

"On 15 March our history changed forever. Now, our laws will too. We are announcing action today on behalf of all New Zealanders to strengthen our gun laws and make our country a safer place," Ms Ardern told a new conference.

“All semi-automatic weapons used during the terrorist attack on Friday 15 March will be banned."

Ms Ardern said she expects the new laws to be in place by April 11 and a buy-back scheme will be established for banned weapons.

The buyback would cost up to NZ$200 million ($138 million), she said.

All military style semi-automatics (MSSA) and assault rifles would be banned, along with parts used to convert weapons into MSSAs and all high-capacity magazines.

Under existing New Zealand gun laws, A-category weapons can be semi-automatic but limited to seven shots. Live-streamed video of a gunman in one of the mosques showed a semi-automatic weapon with a large magazine.

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.

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.

"I strongly believe that the vast majority of legitimate gun owners in New Zealand will understand that these moves are in the national interest, and will take these changes in their stride."

Floral tributes to those who were gunned down at the two mosques are seen against a wall bordering the Botanical Garden in Christchurch Credit: MARTY MELVILLE/AFP

New Zealand, a country of less than 5 million people, has an estimated 1.2-1.5 million firearms, around 13,500 of them MSSA type weapons.

Most farmers in the Pacific country own guns, which they use for killing pests such as possums and rabbits, and for putting down injured stock.

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Recreational hunting of deer, pigs and goats is popular for sport and food, while gun clubs and shooting ranges dot the country.

That has created a powerful lobby which has thwarted previously attempts to tighten gun laws after other mass shootings in New Zealand and overseas.

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

"This will not be popular among some of our members but...we believe this is the only practicable solution," Federated Farmers Rural Security spokesman Miles Anderson said in a statement.

The changes exclude two general classes of firearms which are commonly used for hunting, pest control, stock management on farms, and duck shooting.

"I have a military style weapon. But to be fair, I don't really use it, I don't really need it," said Noel Womersley, who slaughters cattle for small farmers around Christchurch.

"So I'm quite happy to hand mine over, to be fair."

Ms Ardern said the next tranche of reforms will cover the firearm registry and licencing.

Also on Thursday, police said they'd inadvertently charged Tarrant with the murder of a person who is still alive.

Police said in a statement they had apologized to the person incorrectly named on the document and would change the charge sheet. They said the charge remains valid, so there was no chance the suspect would be released as a result of the error.

Police did not offer further details of what went wrong or make anybody available for an interview.

The name of the person on the charging sheet has been suppressed by court order. Officials said more charges against Tarrant would follow.

Tarrant, 28, is next scheduled to appear in court on April 5, and Bush said investigations into him were continuing. Police have said they are certain Tarrant was the only gunman but are still investigating whether he had support.