If you like adventure and discovery only stay a short time on the main tourist trail and try the east coast from Wellington up through Wairarapa, Napier, Gisborne, East Cape and round to Coromandel Peninsula through Auckland quickly then up to Northland. The fishing,surfing etc is sublime this time of year.
The mountains in the centre of the N.I. are breathtaking in Summer(the chair lifts still operate so worth it for the views and you can climb up to the crater from top lift) or Winter(snowboarding etc)
The South Island is like a different country and spectacular in other ways,bigger mountains. Try to be ready to change destination depending on weather,eg the awesome West Coast could be sunny while the east has rain.
There are fantastic hiking trails and hot mineral pools all through both islands for a relaxing soak (some are not obvious).
You will need plenty of time to explore each island and most people I have met in Australia who have been there love it and go back again.
