Magazine capacity limits are relevant in principle, but the problem is enforcement - they're just too simple to effectively regulate. Even defining what a limit is can be hard, given that it's a box with a spring, so the limit inherently depends on what cartridges can fit and feed. As Canada has found out, this means that a 5-round magazine can be a 15-round magazine sometimes. But even when you have a clear definition that accounts for all such shenanigans, good luck enforcing it when one can make high-cap mag bodies by cutting legal ones, and duck taping them together. Again, it's a metal box with a spring, not even curved in many calibers; most complicated parts are the follower and the feed lips, and those are the same regardless of capacity.
I'd still look at their definitions first, though, because in most countries (and US states) that limit magazine capacity, a full-sized magazine that is blocked internally by some kind of pin is perfectly legal, and yet that pin takes about 50 seconds to remove without breaking the magazine. There's a lot more security theater about gun laws in most places than people realize.
I think you'd have said the same thing about New Zealand a week and a half ago, yet here we are.
OTOH, looking at New Zealand's eligibility laws, one thing that stands out to me is the lack of a mandatory mental health exam. So it's like US in that regard - only already-diagnosed mental health issues can block ownership.
At the same time, countries that don't bother with stuff like that, but instead regulate who can own firearms much more stringently than US does - e.g. Czech Republic - are doing fine.