Unfortunately it's a monopoly, so not many options there.

TiVo over here occasionally have sales on HNP to raise money for charity, they may do the same over there in the future. Worth asking support maybe.

Also you might be able to buy one off eBay.au as some people have bought a few when there has been a promotion and try to make a quick buck. But I don't know for sure if the NZ system will accept an AU HNP code.

Peter.