Hi Chris,
I did this trip, Australia and New Zealand, in March/April this year and it truly was amazing, the trip of a lifetime! For a perfect review of the Australian part, see Ivy Bember's review on the Reviews page - she captures it exactly, and the New Zealand trip is well described by Cindy, below.
Just bear in mind that they are both quite different holidays - different Tour Manager and, apart from 4 others who were in Australia with me, a new crowd to get to know. Having bonded with the Oz travellers for 3 weeks, it could be tricky enough starting off again with a new group, but it worked out okay for me as they were all really nice people and good fun, and both Tour Managers (Sarah Wrightson and Brian Croft) were brilliant.
Also, having travelled extensively in Australia for 3 weeks, I was pretty tired on arrival in NZ and facing into lots more long journeys, so please bear that in mind. I suppose if I had to make the decision all over again, I would have done the two bits separately as I did not find the flights a problem and think I would have appreciated NZ more. Still, I did have a wonderful time all round!
That's just my experience, so if you feel like going for both at the same time then good luck and enjoy!!
Marie