New Zealand’s governor-general has formally signed into effect sweeping gun laws outlawing military-style weapons, less than a month after a man used such guns to kill 50 people and wound dozens at two mosques in Christchurch.

Governor-General Patsy Reddy signed the bill on Thursday as police said a gun buyback program will be announced to collect the now-banned weapons.

The weapons will be illegal starting at midnight, but police said a brief amnesty program will be in effect until details of the buyback are announced.

Anyone who retains such a weapon now faces a penalty of up to five years in prison.

Exemptions allow heirloom weapons held by collectors or weapons used for professional pest control.

Australian Brenton Harrison Tarrant, 28, was charged with 50 counts of murder and 39 counts of attempted murder.

The royal commission set up to investigate issues surrounding the massacre is examining how he obtained a gun licence in New Zealand and purchased weapons and ammunition.

Prime Minister Jacinda Ardern spoke emotionally during the bill’s final reading of the traumatic injuries suffered by victims of the March 15 attack, whom she visited in Christchurch Hospital after the shootings.

There is an estimated 1.5 million guns in New Zealand in total. That’s about one for every three citizens and more than double the rate of ownership in Australia.

The law bans a wide range of semi-automatic weapons along with any parts that can convert a lower-powered gun into a semi-automatic.

It also creates new firearms offences – some punishable by 10-year maximum jail sentences – but exempts some weapons such as .22 calibre rifles and shotguns that have smaller ammunition capacity.

Of 120 members in New Zealand’s parliament, only one opposed the changes: The libertarian Act party’s sole MP, David Seymour. He argued the laws have been rushed through too quickly and without enough consultation.