MIGHT is the keyword there
They're not specifically locked. But the devices they sell will be tested, and verified, and OTA approved first
Huawei Pxx range for instance, basically the same handset across all providers with a single-ish update stream. But reset it with a different providers SIM in and it will configure itself during boot and request updates for that provider.
So as long a the device is a known for sale in NZ SKU, with valid OTA updates. It'd probably get it. Get one from china, that has random OS and no real manufaturer OTA updates - less likely