I did two, ~two-week trips in the early 00’s and loved every minute of each. For the first, I flew into Sydney, and the next day, took the train across Australia to Perth for four days. I flew into Melbourne for a ballet performance and wandered for a couple of days, before taking an overnight train to Adelaide, where I stayed for a week to attend the Adelaide Festival and Fringe Festival, then returning to Sydney for a few days before flying home.
For the second trip, I flew to Sydney and left the next night on the overnight ferry to Tasmania — alas, no longer running — for a three -day beach hike and eco-lodge stay along the Bay of Fires, flew to Adelaide for eight days of opera, cricket, and tours to each of the three wine regions outside Adelaide, and then flew to Sydney for a couple of days before flying home.
I could have done either of those trips with one of the two trips I took to NZ in the late ‘90’s, but I would have used commercial transportation, not biking again. I did the north island trip the first time,and the South Island trip the second time.
It depends on how much you want to be in cars to go outside cities and on your own, ie, not part of organized groups or on planes or long train rides, and, depending on how crowded the areas you want to go to, without a rigid itinerary. I planned my trips around fixed events and plane and train tickets, and I don’t travel without advanced hotel reservations, but that’s not other people’s idea of travel.