I am talking merely from a potential medical point of view (injury, sudden illness, etc) since I believe Australians can use New Zealand hospitals without charge

Correct me if I am wrong.

I don't care about other aspects of travel insurance (lost luggage, delays, etc)

Kiwis need insurance when we go to Australia, so I would think it the same the other way.

It's an option my friend if you want to get a travel insurance then you can if you don't then you can.