New Zealand's Government has launched a multi-million-dollar, six-month buyback scheme to compensate owners of semi-automatic weapons, which were newly banned in the wake of deadly attacks on two mosques in Christchurch.

Key points: Almost $200 million has been set aside to compensate firearms owners

Almost $200 million has been set aside to compensate firearms owners They will have until December 20 to hand in their weapons

They will have until December 20 to hand in their weapons Parliament passed the gun reform law by a vote of 119 to 1 in April

Finance Minister Grant Robertson and Minister of Police Stuart Nash said in a joint emailed statement that NZ$208 million ($198 million) had been set aside to compensate owners of the banned semi-automatic firearms up to 95 per cent of the original cost.

They would have until December 20 to hand in their weapons.

"Police have detailed plans in place for the next step, which is the collection of firearms from the community," Mr Nash said.

"It will be a huge logistical exercise and is expected to get underway in mid-July."

Parliament passed the gun reform law — the first substantial changes to the country's gun laws in decades — by a vote of 119 to one in April.

The vote came less than a month after its worst-ever peacetime mass shooting in which 51 people were killed and dozens injured in attacks on two mosques in Christchurch.

Gun laws were swiftly changed after 51 people were shot dead in the Christchurch mosque attacks. ( ABC News: Brendan Esposito )

Australia launched a gun buyback scheme in the wake of the Port Arthur massacre in 1996.

Thousands of military-style weapons covered under scheme

In Australia, a gun buyback scheme was introduced in the wake of the Port Arthur massacre. ( Supplied )

The new ban bars the circulation and use of most semi-automatic firearms, parts that convert firearms into semi-automatic firearms, magazines over a certain capacity and some shotguns.

Existing gun laws had provided for a standard "A-category" gun license covering semi-automatics limited to seven shots.

Police estimated around 14,300 military-style semi-automatics would be covered by the new legislation, though the Government said it was difficult to predict the exact number.

Almost 700 weapons had already been handed in before the compensation scheme was launched and almost 5,000 had been registered by owners with police while they awaited collection.

"There is high uncertainty around any [costs], owing to the lack of information on the number of prohibited items, their type and condition," Mr Robertson said, adding that better estimates of the total cost would be available once the buyback was underway and the Government would top up the amount if needed.

New Zealand Prime Minister Jacinda Ardern flagged immediate changes to gun laws a day after the attacks. ( Supplied: Christchurch City Council )

The Government has also begun work on a second arms amendment bill to tackle other issues including a gun registry.

Prime Minister Jacinda Ardern told reporters on Monday that she expected the Government to announce its plans for the law in the next few weeks.

With a population of just under 5 million, New Zealand is home to an estimated 1.5 million firearms, making it the country with the 17th highest rate of civilian firearm ownership in the world, according to the Small Arms Survey.

Reuters