Howdy Charlotte....You might like to reach a comprimise....and invest in a Camper Trailer (not really seen in NZ yet)
Tenting is great if you plan on staying put for a few days each time....if you buy a 4wd to travel then you will get to see a lot more than in a Campervan as you can get off road and drive into towns/ cities...(easier parking than a Motor-home)
Motor-home wins hands down other-wise when travelling through Aussie...(keeps snakes out as well...lol)
Camper trailers can be a 50/50 of both the above...plus,,,you are sleeping up off the ground.
As Titch asked earlier....where are you coming from ,,,, how long will you be staying...where did you intend to travel to while you are here,,,are you working your way around ???? etc....(wish these questions were in the ''Introduction'' area)
Do some study on our climate for that time of the year...e.g...up here in Queensland its our rain / cyclone season...not the best for tenting. (if you were here to-day...its a chrystal clear beautiful day...not a cloud in the sky and around 21...23deg...not bad for winter (even warmer up North)
Keep in mind that Jan here....will be a lot warmer than in NZ...So you may want to concentrate on the bottom region of Oz.....plus you will be smack in the middle of the school holidays and HIGH tarrifs...if you are going to be staying in camps...then booking ahead will be a MUST.
Cheers & enjoy your trip...
Life is not about how hard you can hit, but how hard you can get hit and keep moving forward.