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.