I really don't believe that NZ beds are harder or softer than their North American equivalents, and wonder how much weight you should apply to internet reviews which will be so personal to the reviewer (probably as much as my opinion here!)...
However, as it sounds like you have quite particular requirements, and if you are staying at hotels and motels, you could contact them directly to alleviate your concerns? For example, there is a hotel chain (Scenic) that uses a (removable) down mattress topper to add a plump, soft, cosy layer (over a normal mattress - ie. what you lie on not what goes over you), which I think are amazing, but you might not and they could remove for you if they knew you weren't a fan.
Edited to add a link - the hotel sells them!
https://www.scenichotelgroup.co.nz/gift-shop/product-detail/bed-topper?productlistPCMID=136883&gclid=EAIaIQobChMI6KTV5sv5-QIVWB0rCh1HEA2gEAAYASAAEgIj6vD_BwE
If we're talking hostels and huts in national parks, then you've probably got what you've got...
Camping mats (of the type you've linked) are obviously available here, they are unlikely to be cheaper but could save having to bring from the US? Camping retailers include www.kathmandu.co.nz or www.bivouac.co.nz or www.macpac.co.nz , or if you want to go really budget www.thewarehouse.co.nz