The problem with pointing to New Zealand for information is they never had widespread community transmission. Australia had the same no mask policy until such a point that they had community transmission and now they are forcing masks where most transmission is occurring.

From the page you link > This advice on use of face masks in the community will be updated if new evidence or guidance from the World Health Organization emerges, or we see a change in COVID-19 disease patterns in New Zealand (for example, increasing community transmission).

So they are simply not advocating masks due to low community transmission.