New Zealanders prepared for nationwide prayers on Friday to mark one week since a mass shooting at two mosques in Christchurch killed 50 worshippers.

Prime Minister Jacinda Ardern will lead thousands of mourners expected to gather at a park in front of the Al Noor mosque, where most of the victims died, for a prayer followed by two minutes of silence.

Ardern, who has labeled the attack as terrorism, announced a ban on military-style semi-automatic and assault rifles under tough new gun laws on Thursday.