As best we presently understand it, New Zealand was never part of the Australian Continental landmass.
New Zealand probably formed in an oceanic trench , between a granite landmass (possibly Queensland), and a volcanic arc.
Eventually, these sediments consolidated and dried out, and gradually rose above the waves. At later times, this new land was connected to S America, and fleetingly to New Caledonia, and perhaps even directly to Australia , but only for short periods of time.
These connections explain the flora and fauna. near Kawhia Harbour, are conglomerates of granite boulders, derived from some distant shore. And lesser amounts at other places as well.