I've done Flinders rangers, withsunday islands, Fraser island, Alice springs (Ayers rock and kaltajuta) and also the great barrier reefs. Awesome i say to all. If you are planning to go to adelaide, you can try hiking Morialta Park. I loved it when i was there.
The tongariro crossing in the North island of NZ is amazing. its got stunning emerald lakes, the red crater, and generally its pretty awesome walking on a dormant volcano with the views and volcanic rock and ash around you. I loved it.
Also Abel Tasman national park in the south isl is stunning. you can get water taxis to certain points and walk back or do the whole 3 day walk. Stewart Island also has some amazing walks i believe although it rained while i was there so i cant speak from first hand experiece.
New Zealand is just an amazing place. I was there for 2 months and that wasnt long enough!
Hope this helps
P.s Wish i was still there!!!!