Who was it that said "never let a good crisis go to waste"... In the case of NZ, there are a few facts to consider. First, there is no actual crisis of gun violence. Less than 10 murders a year are committed with guns of all kinds. Any evidence that the proposed law will have any impact on this already tiny number? Second, the law does NOT ban "military grade weapons" or "weapons of war". Militaries use fully automatic weapons, wouldn't be caught dead (sorry, bad use of phrase) using civilian imitations that are not full auto. These weapons are not available to civilians. Finally, the "if it just saves one life...." argument. It's infantile. The same could be said in favour of banning tobacco, or alcohol, or sugary drinks, or cars, or ... all are easily preventable with a simple act of government...
Not sure about NZ, but in the United States the number of murders committed using rifles of all kinds (semi automatic, "assault rifles", whatever those are, hunting rifles) is statistically insignificant. More people are killed with bare hands than with rifles. More people are killed with clubs and other blunt objects than rifles. The weapon of choice is the handgun, yet nobody proposes any kind if significant regulation on handguns. But what this does tell us is that politicians who want to grandstand without actually solving the problem of gun violence can do so by whining about so-called "assault weapons" and "weapons of war" and gain social justice points, while actually not doing anything to solve the problem. In NZ, what is undeniable is that the murder rate is at its lowest point in 40 years. There have been only 7-10 gun related murders a year for most of the same time period. At least up to this year. So I don't see what a ban on any kind of firearms will do to make it even lower.