The first official flight was to Perth on or around that date, but to allow the staff and pilots time on it they did some 'trial' flights to Sydney and also some that just sat on the ground to help get certification from CAA for it and to sort out issues with services etc prior to the public getting on it.
I'm pretty sure that the latest 773 that Airnz got went into service about 2-3 days after arriving into Auckland, and the 2nd 789 was in service about a week after it turned up.