Have a look at: NZ Adding VoLTE to X10III
Fantastic work by Tom.
tldr: it didn’t have anything to do with those dirs/mbn files. AU is working without them. NZ works when reflashed, then the phone locks out the modem next time it is rebooted. Using SIM2 avoids this.
Quite why it knows to lock out the modem in NZ, but not AU remains unknown.