Prime Minister Jacinda Ardern has announced the Government will ban all military-style semi-automatic weapons and assault rifles, in light of the Christchurch terror attack.

"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," Ardern said on Thursday.

"Cabinet agreed to overhaul the law when it met on Monday, 72 hours after the horrific terrorism act in Christchurch. Now, six days after this attack, we are announcing a ban on all military style semi-automatics (MSSA) and assault rifles in New Zealand."

Ardern first floated the idea of tightening gun laws on Saturday, the morning after the attacks, when 50 people were killed by a suspected white supremacist gunman who wielded legally obtained semi-automatic weapons.

"There have been attempts to change our laws in 2005, 2012 and after an inquiry in 2017. Now is the time for change," she said at the time.

Speaking in Parliament on Thursday, Ardern said none of the changes made in the past dealt with one of the most glaring issues that sets New Zealand apart from many other nations: the availability of military-style semi-automatic weapons.