Wonder if it's the geography
After a stunning holiday in NZ South Island last year I wonder if the problem is due to geography - there is very little room between those mountains. It will be very interesting to see where they claim the interference occurs.
My physics isn't good enough to know if the presence of the mountains, of various rock types, could cause interference between bands that are normally safely mixed in wide-open spaces.
There are very few places in Australia that come close to matching the NZ geography. Maybe if we stretched NZ out to the same size as Oz it would be as flat.