Here is Wikis description of New Zealand gun laws…
https://en.wikipedia.org/wiki/Gun_laws_in_New_Zealand
In New Zealand the emphasis is on vetting the gun owner. If the shooter bought 4 guns as a foreigner in New Zealand he had to have extensive police vetting, in particular with the weapons the shooter bought he would have brought even more scrutiny onto himself from the police due to being classified as Military-style semi-automatics which is a more difficult license to obtain.
Couple that with Skippy and NZ SAS being in the area, as well as how quick this gun grab is going down and can't forget the arrests with out bail for possessing the livestream of the shooting.
https://www.nzherald.co.nz//nz/news/article.cfm?c_id=1&objectid=12213956&ref=clavis