Prime Minister Jacinda Ardern has announced a ban on military style, semi-automatic (MSSA) guns and assault rifles in the wake of the Christchurch mosque shootings.
All semi-automatic weapons used during the terrorist attack on Friday will be banned, she said.
Related parts used to convert the guns into MSSAs would also be banned, along with all high-capacity magazines.
Cabinet had still to consider issues surrounding a gun register, she said.
She also announced immediate action to prevent stockpiling, and a buyback scheme that could cost anywhere between $100m and $200m.
The buyback scheme was being established as an incentive for owners, who legally bought their guns, to return weapons and was being developed alongside the drafting of the new law.
There would be time for the handover of the banned guns to police and it would not be criminalised overnight, she said.
After a "reasonable time" those who continued to possess the guns would be breaking the law. The current fines were up to $4000 and/or three years in prison, but the new law would increase the penalties.
"On 15 March our history changed forever. Now, our laws will too. We are announcing action today [Thursday] on behalf of all New Zealanders to strengthen our gun laws and make our country a safer place," Ardern said.
Speaking on Thursday in Wellington, announcing the stronger gun laws, she said changes to the laws after the Aramoana massacre in 1990, and subsequent additions in following years, did not go far enough.
She reaffirmed loopholes in the current law continued to exist.
Legislation to give effect to the ban will be introduced under urgency when Parliament sits in the first week of April.
A shortened select committee process, for feedback on the technical aspects of the changes, would apply, so she urged those who wished to submit, to start now.
She expected the amendments to the Arms Act to be in place by April 11.
An amnesty will be put in place for weapons to be handed in.
As an interim measure to ensure the trade of the weapons ceased at 3pm on Thursday, the weapons had been re-categorised as requiring an E endorsement on a firearms licence, preventing their sale to people with A category licences.
"This will mean that no-one will be able to buy these weapons without a permit to procure from the police. I can assure people that there is no point in applying for such a permit."
She believed the vast majority of legitimate gun owners in New Zealand would understand the moves were in the national interest, and would take the changes in their stride.
When Australia undertook similar reforms, their approach was to allow for exemptions for farmers upon application, including for pest control and animal welfare.
New Zealand has taken similar action to identify the weapons legitimately required in those areas, and preclude them, she said.
"Be assured this is just the beginning of the work we need to do," Ardern said.
Police Minister Stuart Nash said the Bill would include narrow exemptions for legitimate business use, which would include professional pest control.
Police and the Defence Force will also have exemptions and issues like access for mainstream international sporting competitions were also being worked through, he said.
"We have also acknowledged that some guns serve legitimate purposes in our farming communities, and have therefore set out exemptions for 0.22 calibre rifles and shotguns commonly used for duck hunting. These will have limitations around their capacity."
While the legislation was being drafted, the Government would take immediate action on Thursday to restrict the potential stock-piling of these guns and encourage people to continue to surrender their firearms.
Earlier on Thursday afternoon, an Order in Council of the Arms Act was signed by the Governor-General to reclassify a wider range of semi-automatic weapons under the Act. It came into effect at 3pm Thursday.
"This interim measure will ensure that all of the weapons being banned under amendments to the Arms Act are now categorised as weapons requiring an E endorsement on a firearms licence.
"The effect of this is that it will prevent the sale of MSSAs and assault rifles to people with A category gun licences," he said.
There would be transitionary measures for gun owners to hand in their guns to police to hold until details of a buyback were announced.
"Again, we encourage gun owners to phone in to police ahead of time to advise them they are bringing their guns in to the station," Nash said.
The actions announced would be the first step of the Government's response.
It would be developing stronger and more effective licensing rules, storage requirements and penalties for not complying with gun regulations.
Cabinet agreed to overhaul the law when it met on Monday, 72 hours after the horrific terrorism act in Christchurch.