Prime Minister Jacinda Ardern has vowed to unveil gun law reforms before next Monday.

Cabinet met today and had made an in-principle decision about changes to gun laws.

"As a Cabinet we were absolutely clear, the terror attack on Friday was the worst act of terrorism on our shores," she said.

It is not yet known what the law will entail, but one expect wants to see a formal register brought into effect. 

Defence Force veteran Josh Jacobson told Larry Williams it's time to look at whether current New Zealand firearms legislation is fit for purpose.

He says at the moment, we can only estimate the number of firearms in New Zealand, which could be between 1.5 and 3 million.

"We just don't have the sort of data so that we could have a conversation about this. And the only way we're going to get that data is by bringing in a registration process."

He says the logical response is to reintroduce a registration process for gun owners.

"We register vehicles, boats, dogs. It's hard to see why we would not."

Jacobson says it is too easy for people to modify these guns.

He says you can currently buy an A-category firearm with a standard firearms license that does not have a magazine attached to it with seven rounds.

"It is very, very easy to put a high capacity magazine onto that weapon, and you now have a military style weapon, to which you need a different type of gun license."

Neither the weapon or the magazines are illegal on their own, only when put together.

Jacobson says that our current legislation on semi-automatic weapons is "cosmetic", and legislators need to define weapons more clearly.