New Zealand Prime Minister Jacinda Ardern vowed on Saturday to tighten counter-terrorism laws this month after a knife-wielding militant known to the authorities stabbed and wounded seven people in a supermarket.

Police shot dead the 32-year-old attacker, a Sri Lankan national who had been convicted and imprisoned for about three years before being released in July, moments after he launched his stabbing spree on Friday.

The Counter Terror Legislation Bill criminalises planning and preparation that might lead to terror attack, closing what critics have said has been a loophole allowing plotters to stay free.