The new ban comes under stronger new gun laws following the killing of 50 people in the country’s worst mass shooting.
Ardern said she expects the new law to be in place by April 11 and buy-back scheme will be established for banned weapons.
"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 said.
"Related parts used to convert these guns into MSSAs are also being banned, along with all high-capacity magazines."