New Zealand Prime Minister Jacinda Ardern's swift decision to ban military-style guns in the immediate aftermath of the country's worst massacre has put the spotlight back on the US.

Just days after a terrorist opened fire and killed 50 people at two mosques in Christchurch, Ms Ardern announced New Zealand "will ban all military-style semiautomatic weapons".

An immediate sales ban also went into effect to prevent stockpiling as new laws were rushed through parliament to impose a complete ban on the weapons, according to the Prime Minister.

"The attacker on 15 March took a significant number of lives using primarily two guns," Ms Ardern said on Thursday.

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"They were assault rifles, and they were purchased legally on an A-category gun licence, the standard licence held by gun owners in New Zealand.

"Every semiautomatic weapon used in the terrorist attack on Friday will be banned."

Ms Ardern was largely praised for the decision within New Zealand and Australia, but many people turned to the US — where contentious debate over gun control remains unresolved despite frequent mass killings — for a reaction. And there was no shortage of that.

Democratic presidential candidate Bernie Sanders praised Ms Ardern on Twitter for taking immediate action following the tragedy.

"This is what real action to stop gun violence looks like," Mr Sanders posted.

"We must follow New Zealand's lead, take on the NRA and ban the sale and distribution of assault weapons in the United States."

A representative for the National Rifle Association in the US, Dana Loesch, replied to Mr Sanders' tweet, drawing comparisons between the rights of Americans and New Zealanders.

"The US isn't NZ. While they do not have an inalienable right to bear arms and to self defence, we do," Ms Loesch wrote.

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Since 1970, more than 1.45 million people have died from guns, which is more than in all the wars in American history (1.4 million). Every day in the US, another 100 people die from gun violence and 300 more are injured, and the country now has more guns than people, the New York Times reports.

In the US, about 22 per cent of guns are still acquired without a background check. The House passed a bill last month to require universal background checks, but political analysts say it has little chance in the Senate this year. As it stands, the country's Second Amendment of the United States Constitution reads: "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."

Juliet Lounsbury, 68, of Maine, told news.com.au she was "a true believer in gun control" and the laws were outdated.

"I so hope that they do ban semiautomatic weapons (in New Zealand) and we in America should too," she said.

"It's been out of control for way too long.

"I simply don't see a need to have a semiautomatic weapon."

Ms Lounsbury said American children were "afraid to go to school or church nowadays" because of gun violence.

"They were places where I felt most safe as a child," she said.

"We have to protect our children and the public.

"I honestly feel that a lot of Americans feel this way … but there are many that don't, and I can't make sense out of that.

"Mass shootings are a common occurrence here and around the world … we've got to do something, and banning these semiautomatic weapons is a good place to start."

Pennsylvania resident David Smith, 53, is pro-guns and wants the Second Amendment to remain unchanged.

"All Americans shouldn't have to lose rights because of people who want to kill other people," he told news.com.au.

"Americans who believe New Zealand has done something honourable, and that the US should do it just because they did, are sheep and don't value the American way of life.

"They can move to New Zealand if they love it that much."

HOW IT WILL WORK

There are nearly 250,000 licensed gun owners in New Zealand, which has a population of five million people. Officials estimate there are 1.5 million guns in the country.

Ms Ardern's government has the numbers in Parliament to make the changes, but it is also supported by opposition parties, so passage of the legislation is assured.

New Zealand has no written constitution, so the right to bear arms is not enshrined in any law.

Flowers and messages of support and love for New Zealand's Muslim community have been placed at a police cordon in Deans Ave, Christchurch. Photo / Michael Craig
Flowers and messages of support and love for New Zealand's Muslim community have been placed at a police cordon in Deans Ave, Christchurch. Photo / Michael Craig

Ms Ardern said people could hand over their prohibited guns under an amnesty while officials develop a formal buyback scheme, which could cost up to $NZ200 million. She said there would be "tightly regulated" exemptions for some owners, such as hunters and farmers. The Government said the police and military would be exempt. Access for international shooting competitions would also be considered.

The prohibition includes semiautomatic guns or shotguns that can be used with a detachable magazine that holds more than five rounds. It also applies to accessories used to convert guns into what the Government called "military-style" weapons.

The ban does not apply to guns commonly used by farmers and hunters, including semiautomatic .22 calibre or smaller guns that hold up to 10 rounds, or semi- automatic and pump-action shotguns with non-detachable magazines that hold up to five rounds.

Semiautomatic refers to a firearm's ability to self-load, not only firing a bullet with each trigger pull but also reloading and making the firearm capable of firing again.

Many different types of firearms, from pistols to rifles and shotguns, can be semiautomatic. Semiautomatic rifles like the AR-15 can often be modified with aftermarket parts to fire in fully automatic mode, and instructions can often be found on the internet.