New Zealand will ban all types of semi-automatic weapons used in the Christchurch attacks, PM Jacinda Ardern has said.

The country's gun laws have been in the spotlight since a gunman killed 50 people at two mosques last Friday.

Ms Ardern said she expected new legislation to be in place by 11 April.

She said a buy-back scheme would be set up for banned weapons, and that measures would be imposed to prevent a run on buying before the law comes in.