Umm Australia imposed the ban straightaway (well midnight of the day of announcement)
How do you manage people already in transit? Long haul flights are up to 19 hours to Auckland. They will have no way to self isolate and NZ doesn't physically have enough hotel space.
Australia may have a plan around it but it's more likely to be chaos.
They may have to return to the country they came from. It is not unusual for someone to be refused entry and to go back on the next flight back to their country of origin. Or maybe the government will take then to a quarantine area.