I use a separate SIM for business trips to NZ even though going for only a few days.

You can get them in AKL arrivals.

Same for Australia at MEL or SYD but the choice is better downtown.
The big providers are in every street.

I generally find data very cheap in Australia compared to some other places I go.
(download maps when you're on WiFi and GPS guidance will cost you next to nothing. Even if you don't it's not bad)