Allow plenty of time for NZ.
Obviously Oz is heaps bigger than NZ but, as a lot of people do, don't make the mistake of spending 90% of your time there. There is a lot to see in NZ (especially the South Island) and some fantastic riding both on and off road. I meet so many travellers (some on bikes, some not) who don't leave enough time for it and go home really regretting it.
Don't get me wrong, Oz has plenty of good stuff, but you can tour NZ and see a much wider variety of landscapes, with some breathtaking scenery and you won't use so much fuel on mile after mile of redish brown nothingness. The people are much nicer too, of course.