Well we’d need to see more detail and part of that would be up to NZ. I guess the app might verify that you have taken the test at their accredited laboratory. Having the COVID test results in the app would be pretty pointless if you still had to show all the printed docs at the AU end. The whole point of this app would be to provide clarity to everyone and have the check-in, immigration etc. process as fast as possible so that social distancing and efficiency can be maintained as well as possible.
Ideally the app could be used to find out where you need to get tested, remind you to get tested etc.
The app would presumably start out with a core feature set and depending on funding, complexity etc. may have features added over time.