New Zealand
While South Island has its' attractions I would not dismiss North Island. North of Auckland you can find some of the largest trees in the world, a beach called 90 mile (but it's only 60 miles long) that you can drive on and is actually a designated highway, the Bay of Islands which even though it's a bit of a tourist honey pot is beautiful and has historical significance too and finally a preserved steam railway running up the middle of a street.
East of Auckland is the Coromandel Peninsular. Again a bit of a tourist honey pot but there's a bird sancturay at Miranda on the way there and once on the peninsular there are beaches where you can dig your own hole in the sand to create your own hot bath. We avoided Rotorua and went to Taupo instead for hot springs and the like. If you want an almost perfect cone volcano go to New Plymouth for Mt Egmont. For a town built in 30s Art Deco style (well, rebuilt following a devasting earthquake) go to Napier. If you enjoy eating fish the local fish and chip bars are all pretty good and in Whakatane there's the Wharf Shed which is rather nice and continues to get good reviews.
If you do want to go to South Island remember that there are flights from Australia into Dunedin and Queenstown as well as Christchurch. The RTWs allow breaks in journey so if they don't offer a flight buy one of the local ones they can be surprisingly inexpensive. In South Island Dunedin has Albatross, Penguins, the only Castle in New Zealand and a tourist train ride into the hinterland. Then take a car south along the coast through the Catlins to Invercargill and go to Stewart Island and Ulva Island for a day or two.